diff --git a/CHANGES/8653.bugfix.rst b/CHANGES/8653.bugfix.rst
new file mode 100644
index 00000000000..5c4d66c181f
--- /dev/null
+++ b/CHANGES/8653.bugfix.rst
@@ -0,0 +1 @@
+Fixed multipart reading when stream buffer splits the boundary over several read() calls -- by :user:`Dreamsorcerer`.
diff --git a/aiohttp/multipart.py b/aiohttp/multipart.py
index 71fc2654a1c..26780e3060c 100644
--- a/aiohttp/multipart.py
+++ b/aiohttp/multipart.py
@@ -266,6 +266,7 @@ def __init__(
     ) -> None:
         self.headers = headers
         self._boundary = boundary
+        self._boundary_len = len(boundary) + 2  # Boundary + \r\n
         self._content = content
         self._default_charset = default_charset
         self._at_eof = False
@@ -346,15 +347,25 @@ async def _read_chunk_from_stream(self, size: int) -> bytes:
         # Reads content chunk of body part with unknown length.
         # The Content-Length header for body part is not necessary.
         assert (
-            size >= len(self._boundary) + 2
+            size >= self._boundary_len
         ), "Chunk size must be greater or equal than boundary length + 2"
         first_chunk = self._prev_chunk is None
         if first_chunk:
             self._prev_chunk = await self._content.read(size)
 
-        chunk = await self._content.read(size)
-        self._content_eof += int(self._content.at_eof())
-        assert self._content_eof < 3, "Reading after EOF"
+        chunk = b""
+        # content.read() may return less than size, so we need to loop to ensure
+        # we have enough data to detect the boundary.
+        while len(chunk) < self._boundary_len:
+            chunk += await self._content.read(size)
+            self._content_eof += int(self._content.at_eof())
+            assert self._content_eof < 3, "Reading after EOF"
+            if self._content_eof:
+                break
+        if len(chunk) > size:
+            self._content.unread_data(chunk[size:])
+            chunk = chunk[:size]
+
         assert self._prev_chunk is not None
         window = self._prev_chunk + chunk
         sub = b"\r\n" + self._boundary
diff --git a/tests/test_multipart.py b/tests/test_multipart.py
index 436b70957fa..6fc9fe573ec 100644
--- a/tests/test_multipart.py
+++ b/tests/test_multipart.py
@@ -2,6 +2,7 @@
 import io
 import json
 import pathlib
+import sys
 import zlib
 from unittest import mock
 
@@ -754,6 +755,66 @@ async def test_invalid_boundary(self) -> None:
             with pytest.raises(ValueError):
                 await reader.next()
 
+    @pytest.mark.skipif(sys.version_info < (3, 10), reason="Needs anext()")
+    async def test_read_boundary_across_chunks(self) -> None:
+        class SplitBoundaryStream:
+            def __init__(self) -> None:
+                self.content = [
+                    b"--foobar\r\n\r\n",
+                    b"Hello,\r\n-",
+                    b"-fo",
+                    b"ob",
+                    b"ar\r\n",
+                    b"\r\nwor",
+                    b"ld!",
+                    b"\r\n--f",
+                    b"oobar--",
+                ]
+
+            async def read(self, size=None) -> bytes:
+                chunk = self.content.pop(0)
+                assert len(chunk) <= size
+                return chunk
+
+            def at_eof(self) -> bool:
+                return not self.content
+
+            async def readline(self) -> bytes:
+                line = b""
+                while self.content and b"\n" not in line:
+                    line += self.content.pop(0)
+                line, *extra = line.split(b"\n", maxsplit=1)
+                if extra and extra[0]:
+                    self.content.insert(0, extra[0])
+                return line + b"\n"
+
+            def unread_data(self, data: bytes) -> None:
+                if self.content:
+                    self.content[0] = data + self.content[0]
+                else:
+                    self.content.append(data)
+
+        stream = SplitBoundaryStream()
+        reader = aiohttp.MultipartReader(
+            {CONTENT_TYPE: 'multipart/related;boundary="foobar"'}, stream
+        )
+        part = await anext(reader)
+        result = await part.read_chunk(10)
+        assert result == b"Hello,"
+        result = await part.read_chunk(10)
+        assert result == b""
+        assert part.at_eof()
+
+        part = await anext(reader)
+        result = await part.read_chunk(10)
+        assert result == b"world!"
+        result = await part.read_chunk(10)
+        assert result == b""
+        assert part.at_eof()
+
+        with pytest.raises(StopAsyncIteration):
+            await anext(reader)
+
     async def test_release(self) -> None:
         with Stream(
             newline.join(