diff --git a/CHANGES/8332.bugfix.rst b/CHANGES/8332.bugfix.rst
new file mode 100644
index 00000000000..70cad26b426
--- /dev/null
+++ b/CHANGES/8332.bugfix.rst
@@ -0,0 +1 @@
+Fixed regression with adding Content-Disposition to form-data part after appending to writer -- by :user:`Dreamsorcerer`/:user:`Olegt0rr`.
diff --git a/aiohttp/multipart.py b/aiohttp/multipart.py
index fe3ad6bacaa..6a441ba6456 100644
--- a/aiohttp/multipart.py
+++ b/aiohttp/multipart.py
@@ -875,8 +875,6 @@ def append_payload(self, payload: Payload) -> Payload:
         if self._is_form_data:
             # https://datatracker.ietf.org/doc/html/rfc7578#section-4.7
             # https://datatracker.ietf.org/doc/html/rfc7578#section-4.8
-            assert CONTENT_DISPOSITION in payload.headers
-            assert "name=" in payload.headers[CONTENT_DISPOSITION]
             assert (
                 not {CONTENT_ENCODING, CONTENT_LENGTH, CONTENT_TRANSFER_ENCODING}
                 & payload.headers.keys()
@@ -957,6 +955,11 @@ def size(self) -> Optional[int]:
     async def write(self, writer: Any, close_boundary: bool = True) -> None:
         """Write body."""
         for part, encoding, te_encoding in self._parts:
+            if self._is_form_data:
+                # https://datatracker.ietf.org/doc/html/rfc7578#section-4.2
+                assert CONTENT_DISPOSITION in part.headers
+                assert "name=" in part.headers[CONTENT_DISPOSITION]
+
             await writer.write(b"--" + self._boundary + b"\r\n")
             await writer.write(part._binary_headers)
 
diff --git a/tests/test_multipart.py b/tests/test_multipart.py
index d7e23592a75..79ee464c0e0 100644
--- a/tests/test_multipart.py
+++ b/tests/test_multipart.py
@@ -1165,6 +1165,13 @@ def test_append_multipart(self, writer: Any) -> None:
         part = writer._parts[0][0]
         assert part.headers[CONTENT_TYPE] == "test/passed"
 
+    async def test_set_content_disposition_after_append(self):
+        writer = aiohttp.MultipartWriter("form-data")
+        payload = writer.append("some-data")
+        payload.set_content_disposition("form-data", name="method")
+        assert CONTENT_DISPOSITION in payload.headers
+        assert "name=" in payload.headers[CONTENT_DISPOSITION]
+
     def test_with(self) -> None:
         with aiohttp.MultipartWriter(boundary=":") as writer:
             writer.append("foo")