diff --git a/backend/server.ts b/backend/server.ts
index 8e4f29a0..6427e4b0 100644
--- a/backend/server.ts
+++ b/backend/server.ts
@@ -65,6 +65,8 @@ import { createScheduledReportPoller } from './services/notification/jobs/schedu
 import { createEmailProviderFromEnv } from './services/notification/emailProvider';
 import type { ReportAnalyticsSnapshot } from './services/notification/scheduledReportService';
 import { createRequestObservability } from './shared/middleware/requestObservability';
+import { createRevenueByCurrencyReportService } from './services/analytics/revenueByCurrencyReport';
+import { createRevenueByCurrencyRouter } from './services/analytics/revenueByCurrencyRouter';
 
 export interface StartServerOptions {
   port?: number;
diff --git a/backend/services/analytics/__tests__/revenueByCurrencyReport.test.ts b/backend/services/analytics/__tests__/revenueByCurrencyReport.test.ts
new file mode 100644
index 00000000..48b9c9d1
--- /dev/null
+++ b/backend/services/analytics/__tests__/revenueByCurrencyReport.test.ts
@@ -0,0 +1,351 @@
+/**
+ * Unit tests for revenueByCurrencyReport.ts
+ *
+ * Covers:
+ *  - toMonthlyEquivalent() for every billing cycle
+ *  - generateReport() — grouping, sorting, percentage, totals
+ *  - generateReport() — filters paused/cancelled subscriptions
+ *  - generateReport() — empty input
+ *  - generateReport() — single currency
+ *  - generateReport() — multiple currencies
+ *  - generateReport() — normalises currency to uppercase
+ *  - exportReport() CSV format — headers, data rows, totals row
+ *  - exportReport() JSON format — round-trips to report object
+ *  - exportReport() CSV — special characters escaped correctly
+ */
+
+import { describe, it, expect, beforeEach } from '@jest/globals';
+import {
+  RevenueByCurrencyReportService,
+  createRevenueByCurrencyReportService,
+  toMonthlyEquivalent,
+  type SubscriptionRevenueInput,
+  type RevenueByCurrencyReport,
+} from '../revenueByCurrencyReport';
+
+// ─── Fixtures ─────────────────────────────────────────────────────────────────
+
+const NOW = new Date('2026-03-15T09:00:00.000Z');
+
+function makeService(): RevenueByCurrencyReportService {
+  return createRevenueByCurrencyReportService({ now: () => NOW });
+}
+
+function sub(overrides: Partial<SubscriptionRevenueInput> = {}): SubscriptionRevenueInput {
+  return {
+    amount: 10,
+    currency: 'USD',
+    billingCycle: 'monthly',
+    status: 'active',
+    ...overrides,
+  };
+}
+
+// ─── toMonthlyEquivalent ──────────────────────────────────────────────────────
+
+describe('toMonthlyEquivalent', () => {
+  it('monthly — returns amount unchanged', () => {
+    expect(toMonthlyEquivalent(100, 'monthly')).toBe(100);
+  });
+
+  it('yearly — divides by 12', () => {
+    expect(toMonthlyEquivalent(1200, 'yearly')).toBeCloseTo(100);
+  });
+
+  it('annual — divides by 12', () => {
+    expect(toMonthlyEquivalent(1200, 'annual')).toBeCloseTo(100);
+  });
+
+  it('weekly — multiplies by 52/12', () => {
+    expect(toMonthlyEquivalent(120, 'weekly')).toBeCloseTo((120 * 52) / 12);
+  });
+
+  it('custom — returns amount unchanged', () => {
+    expect(toMonthlyEquivalent(50, 'custom')).toBe(50);
+  });
+
+  it('unknown cycle — returns amount unchanged (safe default)', () => {
+    expect(toMonthlyEquivalent(50, 'quarterly')).toBe(50);
+  });
+
+  it('is case-insensitive', () => {
+    expect(toMonthlyEquivalent(1200, 'YEARLY')).toBeCloseTo(100);
+    expect(toMonthlyEquivalent(1200, 'Yearly')).toBeCloseTo(100);
+  });
+});
+
+// ─── generateReport — empty input ────────────────────────────────────────────
+
+describe('RevenueByCurrencyReportService.generateReport — empty input', () => {
+  it('returns zero totals for an empty subscription list', () => {
+    const svc = makeService();
+    const report = svc.generateReport([], 'merch_1');
+    expect(report.currencies).toHaveLength(0);
+    expect(report.totals.totalSubscriptions).toBe(0);
+    expect(report.totals.totalMonthlyRevenue).toBe(0);
+    expect(report.totals.totalAnnualRevenue).toBe(0);
+    expect(report.totals.currencyCount).toBe(0);
+  });
+
+  it('includes merchantId and generatedAt in the report', () => {
+    const svc = makeService();
+    const report = svc.generateReport([], 'merch_42');
+    expect(report.merchantId).toBe('merch_42');
+    expect(report.generatedAt).toBe(NOW.toISOString());
+  });
+});
+
+// ─── generateReport — filtering by status ────────────────────────────────────
+
+describe('RevenueByCurrencyReportService.generateReport — status filtering', () => {
+  it('excludes cancelled subscriptions', () => {
+    const svc = makeService();
+    const report = svc.generateReport(
+      [sub({ amount: 100, status: 'active' }), sub({ amount: 50, status: 'cancelled' })],
+      'merch_1'
+    );
+    expect(report.totals.totalSubscriptions).toBe(1);
+    expect(report.totals.totalMonthlyRevenue).toBeCloseTo(100);
+  });
+
+  it('excludes paused subscriptions', () => {
+    const svc = makeService();
+    const report = svc.generateReport(
+      [sub({ amount: 100, status: 'active' }), sub({ amount: 50, status: 'paused' })],
+      'merch_1'
+    );
+    expect(report.totals.totalSubscriptions).toBe(1);
+    expect(report.totals.totalMonthlyRevenue).toBeCloseTo(100);
+  });
+
+  it('includes subscriptions with no status field (treated as active)', () => {
+    const svc = makeService();
+    const report = svc.generateReport(
+      [{ amount: 100, currency: 'USD', billingCycle: 'monthly' }],
+      'merch_1'
+    );
+    expect(report.totals.totalSubscriptions).toBe(1);
+  });
+});
+
+// ─── generateReport — single currency ────────────────────────────────────────
+
+describe('RevenueByCurrencyReportService.generateReport — single currency', () => {
+  it('computes monthlyRevenue, annualRevenue, and averagePerSubscription', () => {
+    const svc = makeService();
+    const subs = [
+      sub({ amount: 100, currency: 'USD' }),
+      sub({ amount: 200, currency: 'USD' }),
+    ];
+    const report = svc.generateReport(subs, 'merch_1');
+    expect(report.currencies).toHaveLength(1);
+    const entry = report.currencies[0]!;
+    expect(entry.currency).toBe('USD');
+    expect(entry.subscriptionCount).toBe(2);
+    expect(entry.monthlyRevenue).toBeCloseTo(300);
+    expect(entry.annualRevenue).toBeCloseTo(3600);
+    expect(entry.averagePerSubscription).toBeCloseTo(150);
+    expect(entry.percentage).toBeCloseTo(1);
+  });
+});
+
+// ─── generateReport — multiple currencies ─────────────────────────────────────
+
+describe('RevenueByCurrencyReportService.generateReport — multiple currencies', () => {
+  it('groups by currency and calculates correct percentages', () => {
+    const svc = makeService();
+    const subs = [
+      sub({ amount: 300, currency: 'USD' }),
+      sub({ amount: 100, currency: 'EUR' }),
+      sub({ amount: 100, currency: 'EUR' }),
+    ];
+    const report = svc.generateReport(subs, 'merch_1');
+    expect(report.currencies).toHaveLength(2);
+    const usd = report.currencies.find((c) => c.currency === 'USD')!;
+    const eur = report.currencies.find((c) => c.currency === 'EUR')!;
+
+    expect(usd.monthlyRevenue).toBeCloseTo(300);
+    expect(eur.monthlyRevenue).toBeCloseTo(200);
+    expect(usd.percentage).toBeCloseTo(300 / 500);
+    expect(eur.percentage).toBeCloseTo(200 / 500);
+    // Percentages should sum to ~1
+    expect(usd.percentage + eur.percentage).toBeCloseTo(1);
+  });
+
+  it('sorts currencies by monthly revenue descending', () => {
+    const svc = makeService();
+    const subs = [
+      sub({ amount: 50, currency: 'GBP' }),
+      sub({ amount: 300, currency: 'USD' }),
+      sub({ amount: 150, currency: 'EUR' }),
+    ];
+    const report = svc.generateReport(subs, 'merch_1');
+    const codes = report.currencies.map((c) => c.currency);
+    expect(codes).toEqual(['USD', 'EUR', 'GBP']);
+  });
+
+  it('accumulates totals across all currencies', () => {
+    const svc = makeService();
+    const subs = [
+      sub({ amount: 300, currency: 'USD' }),
+      sub({ amount: 200, currency: 'EUR' }),
+    ];
+    const report = svc.generateReport(subs, 'merch_1');
+    expect(report.totals.totalMonthlyRevenue).toBeCloseTo(500);
+    expect(report.totals.totalAnnualRevenue).toBeCloseTo(6000);
+    expect(report.totals.totalSubscriptions).toBe(2);
+    expect(report.totals.currencyCount).toBe(2);
+  });
+});
+
+// ─── generateReport — currency normalisation ──────────────────────────────────
+
+describe('RevenueByCurrencyReportService.generateReport — currency normalisation', () => {
+  it('normalises currency codes to uppercase and groups them', () => {
+    const svc = makeService();
+    const subs = [
+      sub({ amount: 100, currency: 'usd' }),
+      sub({ amount: 200, currency: 'USD' }),
+      sub({ amount: 50, currency: 'Usd' }),
+    ];
+    const report = svc.generateReport(subs, 'merch_1');
+    expect(report.currencies).toHaveLength(1);
+    expect(report.currencies[0]!.currency).toBe('USD');
+    expect(report.currencies[0]!.monthlyRevenue).toBeCloseTo(350);
+  });
+});
+
+// ─── generateReport — billing cycle normalisation ─────────────────────────────
+
+describe('RevenueByCurrencyReportService.generateReport — billing cycle normalisation', () => {
+  it('normalises annual subscriptions to monthly equivalent', () => {
+    const svc = makeService();
+    const report = svc.generateReport(
+      [sub({ amount: 1200, currency: 'USD', billingCycle: 'yearly' })],
+      'merch_1'
+    );
+    expect(report.currencies[0]!.monthlyRevenue).toBeCloseTo(100);
+    expect(report.currencies[0]!.annualRevenue).toBeCloseTo(1200);
+  });
+
+  it('normalises weekly subscriptions to monthly equivalent', () => {
+    const svc = makeService();
+    const report = svc.generateReport(
+      [sub({ amount: 12, currency: 'USD', billingCycle: 'weekly' })],
+      'merch_1'
+    );
+    const expectedMonthly = (12 * 52) / 12;
+    expect(report.currencies[0]!.monthlyRevenue).toBeCloseTo(expectedMonthly);
+  });
+});
+
+// ─── exportReport — JSON ──────────────────────────────────────────────────────
+
+describe('RevenueByCurrencyReportService.exportReport — JSON', () => {
+  let report: RevenueByCurrencyReport;
+
+  beforeEach(() => {
+    const svc = makeService();
+    report = svc.generateReport(
+      [sub({ amount: 100, currency: 'USD' }), sub({ amount: 50, currency: 'EUR' })],
+      'merch_1'
+    );
+  });
+
+  it('produces valid JSON that round-trips to the report object', () => {
+    const svc = makeService();
+    const json = svc.exportReport(report, 'json');
+    const parsed = JSON.parse(json) as RevenueByCurrencyReport;
+    expect(parsed.merchantId).toBe(report.merchantId);
+    expect(parsed.currencies).toHaveLength(report.currencies.length);
+    expect(parsed.totals.totalMonthlyRevenue).toBeCloseTo(report.totals.totalMonthlyRevenue);
+  });
+
+  it('is pretty-printed (contains newlines)', () => {
+    const svc = makeService();
+    const json = svc.exportReport(report, 'json');
+    expect(json).toContain('\n');
+  });
+});
+
+// ─── exportReport — CSV ───────────────────────────────────────────────────────
+
+describe('RevenueByCurrencyReportService.exportReport — CSV', () => {
+  it('first line is a header row with expected columns', () => {
+    const svc = makeService();
+    const report = svc.generateReport([sub()], 'merch_1');
+    const csv = svc.exportReport(report, 'csv');
+    const firstLine = csv.split('\n')[0]!;
+    expect(firstLine).toContain('Currency');
+    expect(firstLine).toContain('SubscriptionCount');
+    expect(firstLine).toContain('MonthlyRevenue');
+    expect(firstLine).toContain('AnnualRevenue');
+    expect(firstLine).toContain('Percentage');
+  });
+
+  it('has one data row per currency', () => {
+    const svc = makeService();
+    const report = svc.generateReport(
+      [sub({ currency: 'USD' }), sub({ currency: 'EUR' }), sub({ currency: 'GBP' })],
+      'merch_1'
+    );
+    const csv = svc.exportReport(report, 'csv');
+    const lines = csv.split('\n').filter((l) => l.trim().length > 0);
+    // header + 3 currencies + totals = 5 non-empty lines
+    expect(lines).toHaveLength(5);
+  });
+
+  it('includes a totals row with TOTAL label', () => {
+    const svc = makeService();
+    const report = svc.generateReport([sub()], 'merch_1');
+    const csv = svc.exportReport(report, 'csv');
+    expect(csv).toContain('"TOTAL"');
+  });
+
+  it('data row contains the currency code', () => {
+    const svc = makeService();
+    const report = svc.generateReport([sub({ currency: 'XLM' })], 'merch_1');
+    const csv = svc.exportReport(report, 'csv');
+    expect(csv).toContain('"XLM"');
+  });
+
+  it('data row contains the correct subscription count', () => {
+    const svc = makeService();
+    const report = svc.generateReport(
+      [sub({ currency: 'USD' }), sub({ currency: 'USD' }), sub({ currency: 'USD' })],
+      'merch_1'
+    );
+    const csv = svc.exportReport(report, 'csv');
+    expect(csv).toContain('"3"');
+  });
+
+  it('escapes double-quotes in values', () => {
+    // Currency codes are always simple strings, but we can test via the
+    // service directly by manufacturing a report with a currency containing a quote.
+    const svc = makeService();
+    // Build a minimal report manually to test the escape path.
+    const report: RevenueByCurrencyReport = {
+      merchantId: 'test',
+      generatedAt: NOW.toISOString(),
+      currencies: [
+        {
+          currency: 'US"D',
+          subscriptionCount: 1,
+          monthlyRevenue: 10,
+          annualRevenue: 120,
+          averagePerSubscription: 10,
+          percentage: 1,
+        },
+      ],
+      totals: {
+        totalMonthlyRevenue: 10,
+        totalAnnualRevenue: 120,
+        totalSubscriptions: 1,
+        currencyCount: 1,
+      },
+    };
+    const csv = svc.exportReport(report, 'csv');
+    // Double-quote in value should be escaped as ""
+    expect(csv).toContain('US""D');
+  });
+});
diff --git a/backend/services/analytics/revenueByCurrencyReport.ts b/backend/services/analytics/revenueByCurrencyReport.ts
new file mode 100644
index 00000000..c25ab333
--- /dev/null
+++ b/backend/services/analytics/revenueByCurrencyReport.ts
@@ -0,0 +1,229 @@
+/**
+ * RevenueByCurrencyReportService
+ *
+ * Groups subscription revenue by currency, producing per-currency totals
+ * (monthly equivalent, annualised, subscription count, share) that can be
+ * returned as JSON or exported as CSV.
+ *
+ * Design goals (issue #1757)
+ * ──────────────────────────
+ *  - Per-currency totals — each currency gets its own row with MRR, ARR,
+ *    subscription count, average, and percentage share.
+ *  - Exportable — `exportReport(report, format)` produces a CSV or JSON
+ *    string suitable for an HTTP download endpoint.
+ *  - Tested — pure functions, injectable clock, no external dependencies.
+ *
+ * The service accepts a `SubscriptionRevenueInput[]` which maps to both the
+ * frontend `Subscription` type (price / billingCycle) and the backend
+ * repository `Subscription` type (amount / billingCycle). This avoids coupling
+ * to either layer.
+ */
+
+// ─── Public types ─────────────────────────────────────────────────────────────
+
+/** Minimal subscription shape needed for revenue calculation. */
+export interface SubscriptionRevenueInput {
+  /** Subscription amount / price in the subscription's currency. */
+  amount: number;
+  /** ISO 4217 currency code, e.g. "USD", "EUR". */
+  currency: string;
+  /** Billing cycle — used to normalise to a monthly equivalent. */
+  billingCycle: string;
+  /** Only active subscriptions are included in revenue totals. */
+  status?: 'active' | 'paused' | 'cancelled';
+}
+
+/** One row per currency in the report. */
+export interface CurrencyRevenueEntry {
+  currency: string;
+  subscriptionCount: number;
+  /** Monthly recurring revenue in this currency. */
+  monthlyRevenue: number;
+  /** Annualised revenue (monthly × 12) in this currency. */
+  annualRevenue: number;
+  /** Average monthly revenue per subscription. */
+  averagePerSubscription: number;
+  /** Share of total monthly revenue across all currencies (0–1). */
+  percentage: number;
+}
+
+/** Aggregated totals across all currencies. */
+export interface RevenueTotals {
+  /** Sum of monthly revenue across all currencies (mixed currencies). */
+  totalMonthlyRevenue: number;
+  /** Sum of annual revenue across all currencies (mixed currencies). */
+  totalAnnualRevenue: number;
+  /** Total active subscription count across all currencies. */
+  totalSubscriptions: number;
+  /** Number of distinct currencies in the report. */
+  currencyCount: number;
+}
+
+/** The full report returned by `generateReport()`. */
+export interface RevenueByCurrencyReport {
+  merchantId: string;
+  generatedAt: string;
+  currencies: CurrencyRevenueEntry[];
+  totals: RevenueTotals;
+}
+
+export type ExportFormat = 'csv' | 'json';
+
+// ─── Pure helpers ──────────────────────────────────────────────────────────────
+
+/**
+ * Normalise a billing cycle and amount to a monthly equivalent.
+ *
+ *   monthly   → amount as-is
+ *   yearly / annual → amount / 12
+ *   weekly    → amount × 52 / 12
+ *   custom    → amount as-is (assume monthly)
+ */
+export function toMonthlyEquivalent(amount: number, billingCycle: string): number {
+  const normalised = billingCycle.toLowerCase();
+  switch (normalised) {
+    case 'yearly':
+    case 'annual':
+      return amount / 12;
+    case 'weekly':
+      return (amount * 52) / 12;
+    case 'monthly':
+    case 'custom':
+    default:
+      return amount;
+  }
+}
+
+/** Escape a value for CSV, handling commas, quotes, and non-ASCII safely. */
+function csvEscape(value: string | number | null | undefined): string {
+  const text = value === null || value === undefined ? '' : String(value);
+  return `"${text.normalize('NFC').replace(/"/g, '""')}"`;
+}
+
+/** Format a number to 2 decimal places for display. */
+function fmt(n: number): string {
+  return n.toFixed(2);
+}
+
+// ─── Service ──────────────────────────────────────────────────────────────────
+
+export class RevenueByCurrencyReportService {
+  private readonly now: () => Date;
+
+  constructor(options: { now?: () => Date } = {}) {
+    this.now = options.now ?? (() => new Date());
+  }
+
+  /**
+   * Generate a revenue-by-currency report from a list of subscriptions.
+   *
+   * Only subscriptions with status "active" (or undefined status) are included.
+   * The result is sorted by monthly revenue descending so the highest-grossing
+   * currency appears first.
+   */
+  generateReport(
+    subscriptions: SubscriptionRevenueInput[],
+    merchantId: string
+  ): RevenueByCurrencyReport {
+    const active = subscriptions.filter(
+      (s) => s.status === undefined || s.status === 'active'
+    );
+
+    // Group by currency and accumulate.
+    const byCurrency = new Map<string, { count: number; monthly: number }>();
+
+    for (const sub of active) {
+      const currency = (sub.currency ?? 'USD').toUpperCase();
+      const monthly = toMonthlyEquivalent(sub.amount, sub.billingCycle);
+      const entry = byCurrency.get(currency) ?? { count: 0, monthly: 0 };
+      entry.count += 1;
+      entry.monthly += monthly;
+      byCurrency.set(currency, entry);
+    }
+
+    const totalMonthly = [...byCurrency.values()].reduce((sum, e) => sum + e.monthly, 0);
+
+    const currencies: CurrencyRevenueEntry[] = [...byCurrency.entries()]
+      .map(([currency, data]) => ({
+        currency,
+        subscriptionCount: data.count,
+        monthlyRevenue: data.monthly,
+        annualRevenue: data.monthly * 12,
+        averagePerSubscription: data.count > 0 ? data.monthly / data.count : 0,
+        percentage: totalMonthly > 0 ? data.monthly / totalMonthly : 0,
+      }))
+      .sort((a, b) => b.monthlyRevenue - a.monthlyRevenue);
+
+    return {
+      merchantId,
+      generatedAt: this.now().toISOString(),
+      currencies,
+      totals: {
+        totalMonthlyRevenue: totalMonthly,
+        totalAnnualRevenue: totalMonthly * 12,
+        totalSubscriptions: active.length,
+        currencyCount: currencies.length,
+      },
+    };
+  }
+
+  /**
+   * Export a report as CSV or JSON string.
+   *
+   * CSV includes a header row and one data row per currency, followed by
+   * a totals row.  JSON is the full report object pretty-printed.
+   */
+  exportReport(report: RevenueByCurrencyReport, format: ExportFormat): string {
+    if (format === 'json') {
+      return JSON.stringify(report, null, 2);
+    }
+
+    // CSV format
+    const headers = [
+      'Currency',
+      'SubscriptionCount',
+      'MonthlyRevenue',
+      'AnnualRevenue',
+      'AveragePerSubscription',
+      'Percentage',
+    ];
+
+    const rows: string[] = [headers.map(csvEscape).join(',')];
+
+    for (const entry of report.currencies) {
+      rows.push(
+        [
+          csvEscape(entry.currency),
+          csvEscape(entry.subscriptionCount),
+          csvEscape(fmt(entry.monthlyRevenue)),
+          csvEscape(fmt(entry.annualRevenue)),
+          csvEscape(fmt(entry.averagePerSubscription)),
+          csvEscape(fmt(entry.percentage * 100) + '%'),
+        ].join(',')
+      );
+    }
+
+    // Totals row
+    rows.push('');
+    rows.push(
+      [
+        csvEscape('TOTAL'),
+        csvEscape(report.totals.totalSubscriptions),
+        csvEscape(fmt(report.totals.totalMonthlyRevenue)),
+        csvEscape(fmt(report.totals.totalAnnualRevenue)),
+        csvEscape(''),
+        csvEscape(report.totals.currencyCount + ' currencies'),
+      ].join(',')
+    );
+
+    return rows.join('\n');
+  }
+}
+
+// ─── Singleton factory ────────────────────────────────────────────────────────
+
+export function createRevenueByCurrencyReportService(
+  options?: { now?: () => Date }
+): RevenueByCurrencyReportService {
+  return new RevenueByCurrencyReportService(options);
+}
diff --git a/backend/services/analytics/revenueByCurrencyRouter.ts b/backend/services/analytics/revenueByCurrencyRouter.ts
new file mode 100644
index 00000000..71e5ffaa
--- /dev/null
+++ b/backend/services/analytics/revenueByCurrencyRouter.ts
@@ -0,0 +1,109 @@
+/**
+ * revenueByCurrencyRouter
+ *
+ * Plain Node.js HTTP handler for the revenue-by-currency report endpoint.
+ * Mounts under `/reports/revenue-by-currency` in server.ts.
+ *
+ * Routes
+ * ──────
+ *   GET /reports/revenue-by-currency?merchantId=…&format=json  (default)
+ *   GET /reports/revenue-by-currency?merchantId=…&format=csv
+ *
+ * The `format` query parameter selects JSON (default) or CSV export.
+ * CSV responses use `text/csv` content-type with a `Content-Disposition`
+ * header for download. JSON responses use `application/json`.
+ */
+
+import type http from 'node:http';
+import { URL } from 'node:url';
+import type {
+  RevenueByCurrencyReportService,
+  SubscriptionRevenueInput,
+  ExportFormat,
+} from './revenueByCurrencyReport';
+
+// ─── Helpers ──────────────────────────────────────────────────────────────────
+
+function sendJson(res: http.ServerResponse, status: number, body: unknown): void {
+  const json = JSON.stringify(body);
+  res.writeHead(status, { 'Content-Type': 'application/json' });
+  res.end(json);
+}
+
+function sendCsv(res: http.ServerResponse, csv: string, filename: string): void {
+  res.writeHead(200, {
+    'Content-Type': 'text/csv; charset=utf-8',
+    'Content-Disposition': `attachment; filename="${filename}"`,
+  });
+  res.end(csv);
+}
+
+// ─── Types ────────────────────────────────────────────────────────────────────
+
+/**
+ * Factory that returns the subscriptions for a merchant.
+ * In production this would call the subscription repository; in tests a
+ * stub can be injected.
+ */
+export type SubscriptionProvider = (
+  merchantId: string
+) => SubscriptionRevenueInput[] | Promise<SubscriptionRevenueInput[]>;
+
+// ─── Router factory ───────────────────────────────────────────────────────────
+
+export function createRevenueByCurrencyRouter(
+  service: RevenueByCurrencyReportService,
+  subscriptionProvider: SubscriptionProvider
+) {
+  const PREFIX = '/reports/revenue-by-currency';
+
+  /**
+   * Handle a request if it matches the `/reports/revenue-by-currency` prefix.
+   * Returns `true` when handled, `false` to fall through.
+   */
+  return async function handleRevenueByCurrencyRequest(
+    req: http.IncomingMessage,
+    res: http.ServerResponse,
+    pathname: string
+  ): Promise<boolean> {
+    const method = req.method ?? 'GET';
+
+    if (!pathname.startsWith(PREFIX)) return false;
+
+    // Only GET is supported.
+    if (method !== 'GET') {
+      sendJson(res, 405, { error: 'Method not allowed. Use GET.' });
+      return true;
+    }
+
+    const url = new URL(req.url ?? '/', `http://${req.headers.host ?? 'localhost'}`);
+    const merchantId = url.searchParams.get('merchantId') ?? '';
+    const formatParam = url.searchParams.get('format') ?? 'json';
+
+    if (!merchantId) {
+      sendJson(res, 400, { error: 'merchantId query parameter is required' });
+      return true;
+    }
+
+    const format: ExportFormat = formatParam === 'csv' ? 'csv' : 'json';
+
+    try {
+      const subscriptions = await subscriptionProvider(merchantId);
+      const report = service.generateReport(subscriptions, merchantId);
+
+      if (format === 'csv') {
+        const csv = service.exportReport(report, 'csv');
+        const date = new Date().toISOString().slice(0, 10);
+        sendCsv(res, csv, `revenue-by-currency_${merchantId}_${date}.csv`);
+      } else {
+        sendJson(res, 200, report);
+      }
+      return true;
+    } catch (err) {
+      sendJson(res, 500, {
+        error: err instanceof Error ? err.message : String(err),
+      });
+      return true;
+    }
+  };
+}