diff --git a/CHANGES/13415.bugfix.rst b/CHANGES/13415.bugfix.rst
new file mode 100644
index 00000000000..f31b9a63e65
--- /dev/null
+++ b/CHANGES/13415.bugfix.rst
@@ -0,0 +1 @@
+Fixed an integer overflow on too large messages -- by :user:`Dreamsorcerer`.
diff --git a/aiohttp/_websocket/reader_c.pxd b/aiohttp/_websocket/reader_c.pxd
index 94c8f18ef2d..33587831d0c 100644
--- a/aiohttp/_websocket/reader_c.pxd
+++ b/aiohttp/_websocket/reader_c.pxd
@@ -114,6 +114,7 @@ cdef class WebSocketReader:
         has_mask=bint,
         fin=bint,
         had_fragments=Py_ssize_t,
+        partial_len=Py_ssize_t,
         payload_bytearray=bytearray,
     )
     cpdef void _feed_data(self, bytes data) except *
diff --git a/aiohttp/_websocket/reader_py.py b/aiohttp/_websocket/reader_py.py
index 6160206a137..40dda00d039 100644
--- a/aiohttp/_websocket/reader_py.py
+++ b/aiohttp/_websocket/reader_py.py
@@ -466,11 +466,14 @@ def _feed_data(self, data: bytes) -> None:
                     OP_CODE_BINARY,
                     OP_CODE_CONTINUATION,
                 }:
-                    projected_size = self._payload_bytes_to_read + len(self._partial)
-                    if projected_size >= self._max_msg_size:
+                    # partial_len declared in reader_c.pxd to keep it in C.
+                    partial_len = len(self._partial)
+                    # payload_bytes_to_read is a signed 64-bit C value,
+                    # use subtraction here to avoid an integer overflow.
+                    if self._payload_bytes_to_read >= self._max_msg_size - partial_len:
                         raise WebSocketError(
                             WSCloseCode.MESSAGE_TOO_BIG,
-                            f"Message size {projected_size} "
+                            f"Message size {int(self._payload_bytes_to_read) + partial_len} "
                             f"exceeds limit {self._max_msg_size}",
                         )
 
diff --git a/tests/test_websocket_parser.py b/tests/test_websocket_parser.py
index 9e3d08a1649..a2438d461e2 100644
--- a/tests/test_websocket_parser.py
+++ b/tests/test_websocket_parser.py
@@ -828,6 +828,33 @@ def test_msg_too_large_text_after_non_fin_text(out: WebSocketDataQueue) -> None:
     assert ctx.value.code == WSCloseCode.MESSAGE_TOO_BIG
 
 
+def test_msg_too_large_declared_length_near_ssize_t_max(
+    out: WebSocketDataQueue,
+) -> None:
+    # Regression test: the header-time size check compares
+    # `payload_bytes_to_read + len(partial)` against max_msg_size. In the
+    # compiled Cython reader payload_bytes_to_read is a signed 64-bit C
+    # value, and RFC 6455 allows a declared length up to 2**63-1, so a
+    # naive addition can wrap around to a negative number and bypass the
+    # limit entirely once anything is already buffered in `partial`.
+    max_msg_size = 4 * 1024 * 1024
+    parser = WebSocketReader(out, max_msg_size, compress=False, decode_text=True)
+
+    # Buffer one byte in `partial` via a non-fin fragment.
+    first = build_frame(b"a", WSMsgType.TEXT, is_fin=False)
+    parser._feed_data(first)
+
+    # Continuation header alone declares the maximum length a signed
+    # 64-bit payload_bytes_to_read can hold: 2**63-1.
+    header = PACK_LEN3(WSMsgType.CONTINUATION, 127, 2**63 - 1)
+    with pytest.raises(
+        WebSocketError,
+        match=rf"^Message size {2**63} exceeds limit {max_msg_size}$",
+    ) as ctx:
+        parser._feed_data(header)
+    assert ctx.value.code == WSCloseCode.MESSAGE_TOO_BIG
+
+
 @pytest.mark.parametrize(
     "opcode",
     (0x3, 0x4, 0x5, 0x6, 0x7, 0xB, 0xC, 0xD, 0xE, 0xF),