diff --git a/CHANGES/12404.bugfix.rst b/CHANGES/12404.bugfix.rst
new file mode 100644
index 00000000000..1c5e82f8d3a
--- /dev/null
+++ b/CHANGES/12404.bugfix.rst
@@ -0,0 +1,3 @@
+Fixed ``BodyPartReader.read()`` and ``BodyPartReader.read(decode=True)`` returning
+:class:`bytearray` instead of :class:`bytes`, violating the documented API contract.
+-- by :user:`CrepuscularIRIS`.
diff --git a/aiohttp/multipart.py b/aiohttp/multipart.py
index 9d5e5d27b84..4d4eee7887f 100644
--- a/aiohttp/multipart.py
+++ b/aiohttp/multipart.py
@@ -322,8 +322,8 @@ async def read(self, *, decode: bool = False) -> bytes:
                 decoded_data.extend(d)
                 if len(decoded_data) > self._client_max_size:
                     raise self._max_size_error_cls(self._client_max_size)
-            return decoded_data
-        return data
+            return bytes(decoded_data)
+        return bytes(data)
 
     async def read_chunk(self, size: int = chunk_size) -> bytes:
         """Reads body part content chunk of the specified size.
diff --git a/docs/multipart_reference.rst b/docs/multipart_reference.rst
index 41c5942c7d7..fb0c4f2e064 100644
--- a/docs/multipart_reference.rst
+++ b/docs/multipart_reference.rst
@@ -44,7 +44,7 @@ Multipart reference
                           from ``Content-Encoding`` header. If it
                           missed data remains untouched
 
-      :rtype: bytearray
+      :rtype: bytes
 
    .. method:: read_chunk(size=chunk_size)
       :async:
diff --git a/tests/test_multipart.py b/tests/test_multipart.py
index 83046ccc034..db288a97d07 100644
--- a/tests/test_multipart.py
+++ b/tests/test_multipart.py
@@ -185,6 +185,17 @@ async def test_read(self) -> None:
             assert b"Hello, world!" == result
             assert obj.at_eof()
 
+    @pytest.mark.parametrize("kwargs", [{}, {"decode": True}])
+    async def test_read_returns_bytes_not_bytearray(
+        self, kwargs: dict[str, bool]
+    ) -> None:
+        # Regression test for https://github.com/aio-libs/aiohttp/issues/12404
+        with Stream(b"Hello, world!\r\n--:") as stream:
+            d = CIMultiDictProxy[str](CIMultiDict())
+            obj = aiohttp.BodyPartReader(BOUNDARY, d, stream)
+            result = await obj.read(**kwargs)
+        assert isinstance(result, bytes)
+
     async def test_read_chunk_at_eof(self) -> None:
         with Stream(b"--:") as stream:
             d = CIMultiDictProxy[str](CIMultiDict())