diff --git a/changes/4219.bugfix.md b/changes/4219.bugfix.md
new file mode 100644
index 0000000000..728e8a7e3a
--- /dev/null
+++ b/changes/4219.bugfix.md
@@ -0,0 +1,3 @@
+`DefaultChunkKeyEncoding.decode_chunk_key` now validates that a chunk key
+starts with the configured `c<separator>` prefix and raises `ValueError` for
+malformed keys, instead of silently decoding them incorrectly.
diff --git a/src/zarr/core/chunk_key_encodings.py b/src/zarr/core/chunk_key_encodings.py
index 098f2c8981..fb2fd95dee 100644
--- a/src/zarr/core/chunk_key_encodings.py
+++ b/src/zarr/core/chunk_key_encodings.py
@@ -79,7 +79,11 @@ def __post_init__(self) -> None:
     def decode_chunk_key(self, chunk_key: str) -> tuple[int, ...]:
         if chunk_key == "c":
             return ()
-        return tuple(map(int, chunk_key[1:].split(self.separator)))
+        # Strip the "c<sep>" prefix (e.g. "c/" or "c.") before splitting.
+        prefix = "c" + self.separator
+        if chunk_key.startswith(prefix):
+            return tuple(map(int, chunk_key[len(prefix) :].split(self.separator)))
+        raise ValueError(f"Invalid chunk key for default encoding: {chunk_key!r}")
 
     def encode_chunk_key(self, chunk_coords: tuple[int, ...]) -> str:
         return self.separator.join(map(str, ("c",) + chunk_coords))
diff --git a/tests/test_chunk_key_encodings.py b/tests/test_chunk_key_encodings.py
new file mode 100644
index 0000000000..dcc93b9249
--- /dev/null
+++ b/tests/test_chunk_key_encodings.py
@@ -0,0 +1,65 @@
+from __future__ import annotations
+
+import pytest
+
+from zarr.core.chunk_key_encodings import DefaultChunkKeyEncoding, V2ChunkKeyEncoding
+
+
+@pytest.mark.parametrize("separator", ["/", "."])
+@pytest.mark.parametrize(
+    "coords",
+    [(), (0,), (1, 2), (10, 0, 3)],
+)
+def test_default_encoding_round_trips(separator: str, coords: tuple[int, ...]) -> None:
+    """Encoding coordinates and decoding the result returns the coordinates."""
+    encoding = DefaultChunkKeyEncoding(separator=separator)  # type: ignore[arg-type]
+
+    key = encoding.encode_chunk_key(coords)
+    assert encoding.decode_chunk_key(key) == coords
+
+
+@pytest.mark.parametrize("separator", ["/", "."])
+@pytest.mark.parametrize("coords", [(0,), (1, 2), (10, 0, 3)])
+def test_v2_encoding_round_trips(separator: str, coords: tuple[int, ...]) -> None:
+    """The v2 encoding round-trips coordinates for either separator."""
+    encoding = V2ChunkKeyEncoding(separator=separator)  # type: ignore[arg-type]
+
+    key = encoding.encode_chunk_key(coords)
+    assert encoding.decode_chunk_key(key) == coords
+
+
+@pytest.mark.parametrize("separator", ["/", "."])
+def test_v2_zero_dimensional_key_is_ambiguous(separator: str) -> None:
+    """A 0-d v2 array stores its sole chunk under `"0"`, the same key a 1-d
+    array uses for chunk 0, so decoding cannot recover the empty tuple on its
+    own -- the array's dimensionality is what disambiguates it."""
+    encoding = V2ChunkKeyEncoding(separator=separator)  # type: ignore[arg-type]
+
+    assert encoding.encode_chunk_key(()) == "0"
+    assert encoding.decode_chunk_key("0") == (0,)
+
+
+@pytest.mark.parametrize(
+    "chunk_key",
+    [
+        "0/1",  # no "c" prefix at all
+        "c0/1",  # "c" not followed by the separator
+        "x/0/1",  # wrong prefix character
+        "",
+    ],
+)
+def test_default_encoding_rejects_key_without_prefix(chunk_key: str) -> None:
+    """A key that does not carry the `c<separator>` prefix is not a chunk key
+    for this encoding, and must be rejected rather than silently decoded."""
+    encoding = DefaultChunkKeyEncoding(separator="/")
+
+    with pytest.raises(ValueError, match="Invalid chunk key"):
+        encoding.decode_chunk_key(chunk_key)
+
+
+def test_default_encoding_rejects_key_using_the_other_separator() -> None:
+    """A key encoded with `.` is not valid for a `/`-separated encoding."""
+    encoding = DefaultChunkKeyEncoding(separator="/")
+
+    with pytest.raises(ValueError, match="Invalid chunk key"):
+        encoding.decode_chunk_key("c.0.1")