diff --git a/CHANGES/10270.bugfix.rst b/CHANGES/10270.bugfix.rst
new file mode 100644
index 00000000000..e3252464dc8
--- /dev/null
+++ b/CHANGES/10270.bugfix.rst
@@ -0,0 +1,2 @@
+``MultipartForm.decode()`` must follow RFC1341 7.2.1 with a ``CRLF`` after the boundary
+-- by :user:`imnotjames`.
diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index 591142d469e..03e35ce789f 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -175,6 +175,7 @@ Jaesung Lee
 Jake Davis
 Jakob Ackermann
 Jakub Wilk
+James Ward
 Jan Buchar
 Jan Gosmann
 Jarno Elonen
diff --git a/aiohttp/multipart.py b/aiohttp/multipart.py
index dc25a83b17a..052e420a302 100644
--- a/aiohttp/multipart.py
+++ b/aiohttp/multipart.py
@@ -983,7 +983,7 @@ def decode(self, encoding: str = "utf-8", errors: str = "strict") -> str:
         return "".join(
             "--"
             + self.boundary
-            + "\n"
+            + "\r\n"
             + part._binary_headers.decode(encoding, errors)
             + part.decode()
             for part, _e, _te in self._parts
diff --git a/tests/test_web_response.py b/tests/test_web_response.py
index f8cf233e148..493e5e66f27 100644
--- a/tests/test_web_response.py
+++ b/tests/test_web_response.py
@@ -1063,7 +1063,7 @@ def read(self, size: int = -1) -> bytes:
         (BodyPartReader(b"x", CIMultiDictProxy(CIMultiDict()), mock.Mock()), None),
         (
             mpwriter,
-            "--x\nContent-Type: text/plain; charset=utf-8\r\nContent-Length: 4\r\n\r\ntest",
+            "--x\r\nContent-Type: text/plain; charset=utf-8\r\nContent-Length: 4\r\n\r\ntest",
         ),
     ),
 )