diff --git a/playwright/_impl/_fetch.py b/playwright/_impl/_fetch.py
index 094f87297..624d5f1b0 100644
--- a/playwright/_impl/_fetch.py
+++ b/playwright/_impl/_fetch.py
@@ -645,7 +645,12 @@ def is_json_parsable(value: Any) -> bool:
     if not isinstance(value, str):
         return False
     try:
-        json.loads(value)
+        # NaN and Infinity are not valid JSON, unlike what json.loads() accepts by default.
+        json.loads(value, parse_constant=_reject_json_constant)
         return True
-    except json.JSONDecodeError:
+    except ValueError:
         return False
+
+
+def _reject_json_constant(value: str) -> Any:
+    raise ValueError(f"{value} is not valid JSON")
diff --git a/tests/async/test_fetch_global.py b/tests/async/test_fetch_global.py
index cd4538e51..00d5b50bf 100644
--- a/tests/async/test_fetch_global.py
+++ b/tests/async/test_fetch_global.py
@@ -391,6 +391,17 @@ async def test_should_json_stringify_string_body_when_content_type_is_applicatio
         ),
     )
     assert req.post_body == b'"foo"'
+    # https://github.com/microsoft/playwright/issues/43329
+    # NaN is not valid JSON, even though json.loads() accepts it.
+    [req, _] = await asyncio.gather(
+        server.wait_for_request("/empty.html"),
+        request.post(
+            server.EMPTY_PAGE,
+            headers={"content-type": "application/json"},
+            data="NaN",
+        ),
+    )
+    assert req.post_body == b'"NaN"'
     await request.dispose()