diff --git a/playwright/_impl/_network.py b/playwright/_impl/_network.py
index 852b5fac7..721ed5f07 100644
--- a/playwright/_impl/_network.py
+++ b/playwright/_impl/_network.py
@@ -209,10 +209,10 @@ async def sizes(self) -> RequestSizes:
     def post_data(self) -> Optional[str]:
         data = self._fallback_overrides.post_data_buffer
         if data:
-            return data.decode()
+            return data.decode("utf-8", errors="replace")
         base64_post_data = self._initializer.get("postData")
         if base64_post_data is not None:
-            return base64.b64decode(base64_post_data).decode()
+            return base64.b64decode(base64_post_data).decode("utf-8", errors="replace")
         return None
 
     @property
diff --git a/tests/async/test_page_network_request.py b/tests/async/test_page_network_request.py
index bab4d5fd0..04049c857 100644
--- a/tests/async/test_page_network_request.py
+++ b/tests/async/test_page_network_request.py
@@ -82,3 +82,25 @@ async def test_existing_response_should_return_none_before_response_arrives(
     assert captured
     # When the "request" event fires, the response has not been received yet.
     assert captured[0] is None
+
+
+async def test_post_data_non_utf8_bytes_does_not_raise(
+    page: Page, server: Server
+) -> None:
+    # Regression: non-UTF-8 bytes in POST body should not crash post_data
+    # post_data_buffer returns the raw bytes; post_data should return a
+    # best-effort decoded string
+    async with page.expect_request("**/upload") as request_info:
+        await page.evaluate(
+            "() => fetch('/upload', {"
+            "method: 'POST',"
+            "body: new Uint8Array([255, 254, 0, 1])"
+            "})"
+        )
+    request = await request_info.value
+    # post_data must not raise UnicodeDecodeError
+    post_data_str = request.post_data
+    assert isinstance(post_data_str, str)
+    # post_data_buffer returns the exact bytes
+    assert request.post_data_buffer == b'\xff\xfe\x00\x01'
+