diff --git a/CHANGES/12170.misc.rst b/CHANGES/12170.misc.rst
new file mode 100644
index 00000000000..23c63db50e9
--- /dev/null
+++ b/CHANGES/12170.misc.rst
@@ -0,0 +1 @@
+Fixed race condition in ``test_data_file`` on Python 3.14 free-threaded builds -- by :user:`rodrigobnogueira`.
diff --git a/aiohttp/formdata.py b/aiohttp/formdata.py
index 8e9157f8e07..50dae68244f 100644
--- a/aiohttp/formdata.py
+++ b/aiohttp/formdata.py
@@ -1,4 +1,5 @@
 import io
+from collections import deque
 from collections.abc import Iterable
 from typing import Any
 from urllib.parse import urlencode
@@ -81,10 +82,10 @@ def add_field(
         self._fields.append((type_options, headers, value))
 
     def add_fields(self, *fields: Any) -> None:
-        to_add = list(fields)
+        to_add: deque[Any] = deque(fields)
 
         while to_add:
-            rec = to_add.pop(0)
+            rec = to_add.popleft()
 
             if isinstance(rec, io.IOBase):
                 k = guess_filename(rec, "unknown")
diff --git a/tests/test_client_request.py b/tests/test_client_request.py
index fc693fc16c3..054496e6748 100644
--- a/tests/test_client_request.py
+++ b/tests/test_client_request.py
@@ -1352,7 +1352,18 @@ async def test_data_file(
         assert isinstance(req.body, payload.BufferedReaderPayload)
         assert req.headers["TRANSFER-ENCODING"] == "chunked"
 
-        resp = await req._send(conn)
+        original_write_bytes = req._write_bytes
+
+        async def _mock_write_bytes(
+            writer: AbstractStreamWriter, conn: mock.Mock, content_length: int | None
+        ) -> None:
+            # Ensure the task is scheduled so _writer isn't None
+            await asyncio.sleep(0)
+            await original_write_bytes(writer, conn, content_length)
+
+        with mock.patch.object(req, "_write_bytes", _mock_write_bytes):
+            resp = await req._send(conn)
+
         assert asyncio.isfuture(req._writer)
         await resp.wait_for_close()