diff --git a/changelog.d/27.added.md b/changelog.d/27.added.md
new file mode 100644
index 00000000..7be3a867
--- /dev/null
+++ b/changelog.d/27.added.md
@@ -0,0 +1 @@
+Add PostgreSQL constraint support protocol with EXCLUDE and NOT VALID options
\ No newline at end of file
diff --git a/src/rhosocial/activerecord/backend/impl/postgres/dialect.py b/src/rhosocial/activerecord/backend/impl/postgres/dialect.py
index 47289d70..31bcd428 100644
--- a/src/rhosocial/activerecord/backend/impl/postgres/dialect.py
+++ b/src/rhosocial/activerecord/backend/impl/postgres/dialect.py
@@ -35,6 +35,7 @@
     SetOperationMixin,
     TruncateMixin,
     ILIKEMixin,
+    ConstraintMixin,
 )
 from rhosocial.activerecord.backend.dialect.protocols import (
     CTESupport,
@@ -65,6 +66,7 @@
     ILIKESupport,
     IntrospectionSupport,
     TransactionControlSupport,
+    ConstraintSupport,
 )
 from .mixins import (
     PostgresExtensionMixin,
@@ -92,6 +94,7 @@
     PostgresTriggerMixin,
     PostgresCommentMixin,
     PostgresTypeMixin,
+    PostgresConstraintMixin,
     # Type mixins
     EnumTypeMixin,
     TypesDataTypeMixin,
@@ -131,6 +134,7 @@
     PostgresTriggerSupport,
     PostgresCommentSupport,
     PostgresTypeSupport,
+    PostgresConstraintSupport,
     # Type feature protocols
     MultirangeSupport,
     EnumTypeSupport,
@@ -189,6 +193,7 @@ class PostgresDialect(
     IndexMixin,
     SequenceMixin,
     TableMixin,
+    ConstraintMixin,
     # Introspection capability
     PostgresIntrospectionCapabilityMixin,
     # PostgreSQL-specific mixins
@@ -217,6 +222,7 @@ class PostgresDialect(
     PostgresTriggerMixin,
     PostgresCommentMixin,
     PostgresTypeMixin,
+    PostgresConstraintMixin,
     # Type mixins
     EnumTypeMixin,
     TypesDataTypeMixin,
@@ -252,6 +258,7 @@ class PostgresDialect(
     IndexSupport,
     SequenceSupport,
     TableSupport,
+    ConstraintSupport,
     # Introspection protocol
     IntrospectionSupport,
     # Transaction control protocol
@@ -282,6 +289,7 @@ class PostgresDialect(
     PostgresTriggerSupport,
     PostgresCommentSupport,
     PostgresTypeSupport,
+    PostgresConstraintSupport,
     # Type feature protocols
     MultirangeSupport,
     EnumTypeSupport,
@@ -890,6 +898,16 @@ def supports_table_tablespace(self) -> bool:
         """Whether tablespace specification is supported."""
         return True
 
+    # Constraint capability overrides
+
+    def supports_constraint_enforced(self) -> bool:
+        """PostgreSQL does not support ENFORCED/NOT ENFORCED (SQL:2016).
+
+        PostgreSQL uses NOT VALID as a proprietary alternative for
+        skipping validation of existing data when adding constraints.
+        """
+        return False
+
     def format_create_table_statement(self, expr: "CreateTableExpression") -> Tuple[str, tuple]:
         """
         Format CREATE TABLE statement for PostgreSQL, including LIKE syntax support.
@@ -1088,6 +1106,118 @@ def format_cast_expression(
 
     # endregion
 
+    # region Constraint DDL Support (PostgreSQL-specific)
+
+    def format_add_table_constraint_action(
+        self, action: "AddTableConstraint",
+    ) -> Tuple[str, tuple]:
+        """Format ADD CONSTRAINT action with PostgreSQL-specific extensions.
+
+        Extends the base class implementation with:
+        - EXCLUDE constraint support (PG-specific constraint type)
+        - NOT VALID suffix (PG-specific: skip validation of existing rows)
+        """
+        from rhosocial.activerecord.backend.expression.statements import (
+            TableConstraintType, ConstraintValidation,
+        )
+
+        # Handle EXCLUDE constraint (PG-specific, not in base class)
+        if action.constraint.constraint_type == TableConstraintType.EXCLUDE:
+            parts = []
+            exclude_sql, params = self._format_exclude_constraint(action.constraint)
+            parts.append(exclude_sql)
+
+            # NOT VALID suffix
+            if action.constraint.dialect_options:
+                validation = action.constraint.dialect_options.get('validation')
+                if validation == ConstraintValidation.NOVALIDATE:
+                    parts.append("NOT VALID")
+
+            return f"ADD {' '.join(parts)}", tuple(params)
+
+        # Use base class for standard formatting (includes DEFERRABLE)
+        sql, params = super().format_add_table_constraint_action(action)
+
+        # PostgreSQL NOT VALID suffix
+        if action.constraint.dialect_options:
+            validation = action.constraint.dialect_options.get('validation')
+            if validation == ConstraintValidation.NOVALIDATE:
+                sql += " NOT VALID"
+
+        return sql, params
+
+    def _format_exclude_constraint(
+        self, constraint: "TableConstraint",
+    ) -> Tuple[str, tuple]:
+        """Format EXCLUDE constraint (PostgreSQL-specific).
+
+        EXCLUDE constraints use the dialect_options dict to specify:
+        - 'exclude_elements': List of (expression, operator) tuples
+          e.g., [('range', '&&')] for EXCLUDE USING gist (range WITH &&)
+        - 'using': The index access method (default 'gist')
+          e.g., 'gist', 'btree', 'spgist'
+        - 'where': Optional predicate for partial exclusion constraints
+
+        Example:
+            TableConstraint(
+                constraint_type=TableConstraintType.EXCLUDE,
+                name='exclude_range_overlap',
+                dialect_options={
+                    'exclude_elements': [('range', '&&')],
+                    'using': 'gist',
+                }
+            )
+            # Generates: EXCLUDE USING gist (range WITH &&)
+        """
+        parts = []
+        params: list = []
+
+        if constraint.name:
+            parts.append(f"CONSTRAINT {self.format_identifier(constraint.name)}")
+
+        # USING clause
+        using = 'gist'  # default
+        if constraint.dialect_options and 'using' in constraint.dialect_options:
+            using = constraint.dialect_options['using']
+        parts.append(f"EXCLUDE USING {using}")
+
+        # Elements: (expression, operator) pairs
+        exclude_elements = []
+        if constraint.dialect_options and 'exclude_elements' in constraint.dialect_options:
+            for expr, op in constraint.dialect_options['exclude_elements']:
+                if isinstance(expr, str):
+                    exclude_elements.append(f"{self.format_identifier(expr)} WITH {op}")
+                else:
+                    # expr is a BaseExpression
+                    expr_sql, expr_params = expr.to_sql()
+                    params.extend(expr_params)
+                    exclude_elements.append(f"{expr_sql} WITH {op}")
+
+        if exclude_elements:
+            parts.append(f"({', '.join(exclude_elements)})")
+
+        # WHERE clause for partial exclusion constraint
+        if constraint.dialect_options and 'where' in constraint.dialect_options:
+            where_expr = constraint.dialect_options['where']
+            where_sql, where_params = where_expr.to_sql()
+            params.extend(where_params)
+            parts.append(f"WHERE ({where_sql})")
+
+        # DEFERRABLE / NOT DEFERRABLE
+        if constraint.deferrable is True:
+            if constraint.initially_deferred is True:
+                parts.append("DEFERRABLE INITIALLY DEFERRED")
+            elif constraint.initially_deferred is False:
+                parts.append("DEFERRABLE INITIALLY IMMEDIATE")
+            else:
+                parts.append("DEFERRABLE")
+        elif constraint.deferrable is False:
+            parts.append("NOT DEFERRABLE")
+
+        return ' '.join(parts), tuple(params)
+
+    # endregion
+
     # region Trigger Support (PostgreSQL-specific)
     def supports_trigger(self) -> bool:
         return True
diff --git a/src/rhosocial/activerecord/backend/impl/postgres/mixins/__init__.py b/src/rhosocial/activerecord/backend/impl/postgres/mixins/__init__.py
index fe2be7ba..07c80018 100644
--- a/src/rhosocial/activerecord/backend/impl/postgres/mixins/__init__.py
+++ b/src/rhosocial/activerecord/backend/impl/postgres/mixins/__init__.py
@@ -21,6 +21,7 @@
 from .ddl.trigger import PostgresTriggerMixin
 from .ddl.comment import PostgresCommentMixin
 from .ddl.type import PostgresTypeMixin
+from .ddl.constraint import PostgresConstraintMixin
 
 # DML mixins
 from .dml.vacuum import PostgresVacuumMixin
diff --git a/src/rhosocial/activerecord/backend/impl/postgres/mixins/ddl/__init__.py b/src/rhosocial/activerecord/backend/impl/postgres/mixins/ddl/__init__.py
index 59910b89..e4ed4e7a 100644
--- a/src/rhosocial/activerecord/backend/impl/postgres/mixins/ddl/__init__.py
+++ b/src/rhosocial/activerecord/backend/impl/postgres/mixins/ddl/__init__.py
@@ -6,6 +6,7 @@
 from .trigger import PostgresTriggerMixin
 from .comment import PostgresCommentMixin
 from .type import PostgresTypeMixin
+from .constraint import PostgresConstraintMixin
 
 __all__ = [
     "PostgresPartitionMixin",
@@ -13,4 +14,5 @@
     "PostgresTriggerMixin",
     "PostgresCommentMixin",
     "PostgresTypeMixin",
+    "PostgresConstraintMixin",
 ]
diff --git a/src/rhosocial/activerecord/backend/impl/postgres/mixins/ddl/constraint.py b/src/rhosocial/activerecord/backend/impl/postgres/mixins/ddl/constraint.py
new file mode 100644
index 00000000..a936f54e
--- /dev/null
+++ b/src/rhosocial/activerecord/backend/impl/postgres/mixins/ddl/constraint.py
@@ -0,0 +1,43 @@
+# src/rhosocial/activerecord/backend/impl/postgres/mixins/ddl/constraint.py
+"""PostgreSQL-specific constraint capability defaults.
+
+Provides default implementations for PostgreSQL-proprietary constraint
+features that are not part of the SQL standard.
+
+SQL standard constraint capability defaults are defined in the core
+ConstraintMixin (rhosocial.activerecord.backend.dialect.mixins).
+
+Note: The format_add_table_constraint_action method is defined directly
+in PostgresDialect rather than in this mixin, because Python's C3 MRO
+places SQLDialectBase before mixins in the inheritance chain, which
+prevents mixin methods from overriding base class methods of the same name.
+This mixin provides capability detection defaults only.
+"""
+
+
+class PostgresConstraintMixin:
+    """PostgreSQL-proprietary constraint capability defaults.
+
+    These features are PostgreSQL-specific and not part of the SQL standard.
+    Both NOT VALID and EXCLUDE constraints are supported by all PostgreSQL
+    versions, so they default to True.
+
+    SQL standard constraint capability defaults are in ConstraintMixin.
+    """
+
+    def supports_constraint_novalidate(self) -> bool:
+        """NOT VALID constraint option (PostgreSQL-proprietary).
+
+        Allows adding a constraint without validating existing data.
+        Supported by all PostgreSQL versions.
+        """
+        return True
+
+    def supports_exclude_constraint(self) -> bool:
+        """EXCLUDE constraints (PostgreSQL-proprietary).
+
+        Exclusion constraints prevent overlapping rows on specified
+        columns using specified operators.
+        Supported by all PostgreSQL versions.
+        """
+        return True
diff --git a/src/rhosocial/activerecord/backend/impl/postgres/protocols/__init__.py b/src/rhosocial/activerecord/backend/impl/postgres/protocols/__init__.py
index fa6d1adf..56ab0f7c 100644
--- a/src/rhosocial/activerecord/backend/impl/postgres/protocols/__init__.py
+++ b/src/rhosocial/activerecord/backend/impl/postgres/protocols/__init__.py
@@ -20,6 +20,7 @@
 from .ddl.trigger import PostgresTriggerSupport
 from .ddl.comment import PostgresCommentSupport
 from .ddl.type import PostgresTypeSupport
+from .ddl.constraint import PostgresConstraintSupport
 
 # DML protocols
 from .dml.vacuum import PostgresVacuumSupport
@@ -62,6 +63,7 @@
     "PostgresTriggerSupport",
     "PostgresCommentSupport",
     "PostgresTypeSupport",
+    "PostgresConstraintSupport",
     # DML protocols
     "PostgresVacuumSupport",
     "PostgresStoredProcedureSupport",
diff --git a/src/rhosocial/activerecord/backend/impl/postgres/protocols/ddl/__init__.py b/src/rhosocial/activerecord/backend/impl/postgres/protocols/ddl/__init__.py
index b32febca..b35ad690 100644
--- a/src/rhosocial/activerecord/backend/impl/postgres/protocols/ddl/__init__.py
+++ b/src/rhosocial/activerecord/backend/impl/postgres/protocols/ddl/__init__.py
@@ -6,6 +6,7 @@
 from .trigger import PostgresTriggerSupport
 from .comment import PostgresCommentSupport
 from .type import PostgresTypeSupport
+from .constraint import PostgresConstraintSupport
 
 __all__ = [
     "PostgresPartitionSupport",
@@ -13,4 +14,5 @@
     "PostgresTriggerSupport",
     "PostgresCommentSupport",
     "PostgresTypeSupport",
+    "PostgresConstraintSupport",
 ]
diff --git a/src/rhosocial/activerecord/backend/impl/postgres/protocols/ddl/constraint.py b/src/rhosocial/activerecord/backend/impl/postgres/protocols/ddl/constraint.py
new file mode 100644
index 00000000..b18f8a5e
--- /dev/null
+++ b/src/rhosocial/activerecord/backend/impl/postgres/protocols/ddl/constraint.py
@@ -0,0 +1,55 @@
+# src/rhosocial/activerecord/backend/impl/postgres/protocols/ddl/constraint.py
+"""PostgreSQL-proprietary constraint capability protocol definitions.
+
+This module contains the PostgresConstraintSupport protocol which defines
+the interface for PostgreSQL's proprietary constraint features that are
+not part of the SQL standard.
+
+SQL standard constraint capabilities are defined in the core
+ConstraintSupport protocol (rhosocial.activerecord.backend.dialect.protocols).
+
+PostgreSQL-proprietary features:
+- NOT VALID (add constraint without validating existing data)
+- EXCLUDE constraints
+"""
+
+from typing import Protocol, runtime_checkable
+
+
+@runtime_checkable
+class PostgresConstraintSupport(Protocol):
+    """PostgreSQL-proprietary constraint capabilities protocol.
+
+    These features are PostgreSQL-specific and not part of the SQL standard.
+    SQL standard constraint features (PRIMARY KEY, UNIQUE, CHECK, FK, etc.)
+    are defined in the core ConstraintSupport protocol.
+
+    Feature Source: Native support (no extension required)
+
+    Official Documentation:
+    - NOT VALID: https://www.postgresql.org/docs/current/sql-altertable.html
+    - EXCLUDE: https://www.postgresql.org/docs/current/ddl-constraints.html#DDL-CONSTRAINTS-EXCLUSION
+    """
+
+    def supports_constraint_novalidate(self) -> bool:
+        """Whether NOT VALID constraint option is supported.
+
+        PostgreSQL-proprietary feature. Allows adding a constraint
+        without validating existing data, then validating later
+        with VALIDATE CONSTRAINT.
+
+        Native feature, all PostgreSQL versions.
+        """
+        ...
+
+    def supports_exclude_constraint(self) -> bool:
+        """Whether EXCLUDE constraints are supported.
+
+        PostgreSQL-proprietary feature. Exclusion constraints ensure
+        that no two rows overlap on specified columns using specified
+        operators (e.g., && for range overlap).
+
+        Native feature, all PostgreSQL versions.
+        Requires appropriate operator class (e.g., btree_gist for gist).
+        """
+        ...