diff --git a/src/openai/_base_client.py b/src/openai/_base_client.py
index 7f92a61a86..3c4f6d1f0b 100644
--- a/src/openai/_base_client.py
+++ b/src/openai/_base_client.py
@@ -591,7 +591,8 @@ def _build_request(
             elif not files:
                 # Don't set content when JSON is sent as multipart/form-data,
                 # since httpx's content param overrides other body arguments
-                kwargs["content"] = openapi_dumps(json_data) if is_given(json_data) and json_data is not None else None
+                if is_given(json_data) and json_data is not None:
+                    kwargs["content"] = openapi_dumps(json_data)
             kwargs["files"] = files
         else:
             headers.pop("Content-Type", None)
@@ -1617,6 +1618,114 @@ async def _send_request(
     ) -> httpx2.Response:
         return await self._client.send(request, stream=stream, **kwargs)
 
+    async def _build_request_async(
+        self,
+        options: FinalRequestOptions,
+        *,
+        retries_taken: int = 0,
+    ) -> httpx2.Request:
+        """Async-safe version of _build_request that runs JSON serialization in a thread pool."""
+        # Request bodies, files, URLs, and custom options can contain private data.
+        log.debug(
+            "Building HTTP request: method=%s retries_taken=%i",
+            get_http_method_for_logging(options.method),
+            retries_taken,
+        )
+        kwargs: dict[str, Any] = {}
+
+        json_data = options.json_data
+        if options.extra_json is not None:
+            if json_data is None:
+                json_data = cast(Body, options.extra_json)
+            elif is_mapping(json_data):
+                json_data = _merge_mappings(json_data, options.extra_json)
+            else:
+                raise RuntimeError(f"Unexpected JSON data type, {type(json_data)}, cannot merge with `extra_body`")
+
+        headers = self._build_headers(options, retries_taken=retries_taken)
+        params = _merge_mappings({**self._auth_query(options.security), **self.default_query}, options.params)
+        content_type = headers.get("Content-Type")
+        files = options.files
+
+        # If the given Content-Type header is multipart/form-data then it
+        # has to be removed so that httpx can generate the header with
+        # additional information for us as it has to be in this form
+        # for the server to be able to correctly parse the request:
+        # multipart/form-data; boundary=---abc--
+        if content_type is not None and content_type.startswith("multipart/form-data"):
+            if "boundary" not in content_type:
+                # only remove the header if the boundary hasn't been explicitly set
+                # as the caller doesn't want httpx to come up with their own boundary
+                headers.pop("Content-Type")
+
+            # As we are now sending multipart/form-data instead of application/json
+            # we need to tell httpx to use it, https://www.python-httpx.org/advanced/clients/#multipart-file-encoding
+            if json_data:
+                if not is_dict(json_data):
+                    raise TypeError(
+                        f"Expected query input to be a dictionary for multipart requests but got {type(json_data)} instead."
+                    )
+                kwargs["data"] = self._serialize_multipartform(json_data)
+
+            # httpx determines whether or not to send a "multipart/form-data"
+            # request based on the truthiness of the "files" argument.
+            # This gets around that issue by generating a dict value that
+            # evaluates to true.
+            #
+            # https://github.com/encode/httpx/discussions/2399#discussioncomment-3814186
+            if not files:
+                files = cast(HttpxRequestFiles, ForceMultipartDict())
+
+        prepared_url = self._prepare_url(options.url)
+        # preserve hard-coded query params from the url
+        if params and prepared_url.query:
+            params = {**dict(prepared_url.params.items()), **params}
+            prepared_url = prepared_url.copy_with(raw_path=prepared_url.raw_path.split(b"?", 1)[0])
+
+        is_body_allowed = options.method.lower() != "get"
+
+        if is_body_allowed:
+            if options.content is not None and json_data is not None:
+                raise TypeError("Passing both `content` and `json_data` is not supported")
+            if options.content is not None and files is not None:
+                raise TypeError("Passing both `content` and `files` is not supported")
+            if options.content is not None:
+                kwargs["content"] = options.content
+            elif isinstance(json_data, bytes):
+                kwargs["content"] = json_data
+            elif not files:
+                # Don't set content when JSON is sent as multipart/form-data,
+                # since httpx's content param overrides other body arguments
+                if is_given(json_data) and json_data is not None:
+                    # Use async serialization to avoid blocking the event loop
+                    kwargs["content"] = await asyncify(openapi_dumps)(json_data)
+            kwargs["files"] = files
+        else:
+            headers.pop("Content-Type", None)
+            kwargs.pop("data", None)
+
+        timeout = self.timeout if isinstance(options.timeout, NotGiven) else options.timeout
+        request_url = str(prepared_url)
+        request_headers = list(headers.multi_items())
+        if is_legacy_httpx_sync_client(self._client) or is_legacy_httpx_async_client(self._client):
+            timeout = normalize_legacy_httpx_timeout(timeout)
+        else:
+            timeout = normalize_httpx2_timeout(timeout)
+
+        # TODO: report this error to httpx
+        return self._client.build_request(  # pyright: ignore[reportUnknownMemberType]
+            headers=request_headers,
+            timeout=timeout,
+            method=options.method,
+            url=request_url,
+            # the `Query` type that we use is incompatible with qs'
+            # `Params` type as it needs to be typed as `Mapping[str, object]`
+            # so that passing a `TypedDict` doesn't cause an error.
+            # https://github.com/microsoft/pyright/issues/3526#event-6715453066
+            params=self.qs.stringify(cast(Mapping[str, Any], params)) if params else None,
+            **kwargs,
+        )
+
     @overload
     async def request(
         self,
@@ -1678,7 +1787,7 @@ async def request(
             options = await self._prepare_options(options)
 
             remaining_retries = max_retries - retries_taken
-            request = self._build_request(options, retries_taken=retries_taken)
+            request = await self._build_request_async(options, retries_taken=retries_taken)
             await self._prepare_request(request)
 
             kwargs: HttpxSendArgs = {}
diff --git a/src/openai/lib/azure.py b/src/openai/lib/azure.py
index c7e61767a2..671fa2c731 100644
--- a/src/openai/lib/azure.py
+++ b/src/openai/lib/azure.py
@@ -155,6 +155,24 @@ def _build_request(
         request.extensions[_AZURE_AUTH_ORIGIN] = _origin(request.url)
         return request
 
+    async def _build_request_async(
+        self,
+        options: FinalRequestOptions,
+        *,
+        retries_taken: int = 0,
+    ) -> httpx2.Request:
+        """Async variant of _build_request for use in async contexts."""
+        if options.url in _deployments_endpoints and is_mapping(options.json_data):
+            model = options.json_data.get("model")
+            if model is not None and "/deployments" not in str(self.base_url.path):
+                options.url = path_template("/deployments/{model}", model=model) + options.url
+
+        request = await super()._build_request_async(options, retries_taken=retries_taken)
+        # HTTPX preserves request extensions through redirects. Scope the hook
+        # to this Azure request, including when its HTTP client is shared.
+        request.extensions[_AZURE_AUTH_ORIGIN] = _origin(request.url)
+        return request
+
     @override
     def _prepare_url(self, url: str) -> httpx2.URL:
         """Adjust the URL if the client was configured with an Azure endpoint + deployment
diff --git a/tests/test_event_loop_blocking.py b/tests/test_event_loop_blocking.py
new file mode 100644
index 0000000000..f0ea251288
--- /dev/null
+++ b/tests/test_event_loop_blocking.py
@@ -0,0 +1,105 @@
+"""Regression tests for event loop blocking during JSON serialization.
+
+This test verifies that async requests don't block the event loop during JSON
+serialization, which is critical for concurrent operations like Redis, Kafka,
+and WebSocket communication that share the event loop.
+
+See: https://github.com/openai/openai-python/issues/3777
+"""
+
+from __future__ import annotations
+
+import asyncio
+from typing import AsyncIterator
+
+import httpx2
+import pytest
+
+from openai import AsyncOpenAI
+from tests.respx2 import MockRouter
+
+
+@pytest.mark.asyncio
+async def test_async_request_does_not_block_event_loop(
+    respx2_mock: MockRouter,
+    async_client: AsyncOpenAI,
+) -> None:
+    """Test that async JSON serialization doesn't block the event loop.
+    
+    This test verifies the fix for issue #3777 by ensuring that:
+    1. Background concurrent work completes while serialization happens
+    2. The event loop remains responsive during JSON serialization
+    3. Multiple concurrent tasks can progress simultaneously
+    """
+    # Track when the background task completes
+    background_task_started = asyncio.Event()
+    background_task_done = asyncio.Event()
+    background_task_iterations = 0
+    
+    async def background_work() -> None:
+        """Simulates concurrent work (e.g., Redis access, WebSocket read)."""
+        nonlocal background_task_iterations
+        background_task_started.set()
+        
+        # Run for a short time to give the event loop a chance to be blocked
+        for _ in range(100):
+            background_task_iterations += 1
+            await asyncio.sleep(0.001)  # 1ms per iteration = 100ms total
+        
+        background_task_done.set()
+    
+    # Setup the mock to return a successful response
+    respx2_mock.post("/chat/completions").mock(
+        return_value=httpx2.Response(
+            status_code=200,
+            json={
+                "id": "chatcmpl-test",
+                "object": "chat.completion",
+                "created": 1234567890,
+                "model": "gpt-4",
+                "choices": [
+                    {
+                        "index": 0,
+                        "message": {"role": "assistant", "content": "Hello!"},
+                        "finish_reason": "stop",
+                    }
+                ],
+                "usage": {"prompt_tokens": 10, "completion_tokens": 5, "total_tokens": 15},
+            },
+        )
+    )
+    
+    # Start the background task
+    background_task = asyncio.create_task(background_work())
+    
+    # Wait for background task to start
+    await asyncio.wait_for(background_task_started.wait(), timeout=1.0)
+    
+    # Make an async API request (which triggers JSON serialization)
+    # This should NOT block the event loop, allowing background_work to continue
+    response = await async_client.chat.completions.create(
+        model="gpt-4",
+        messages=[{"role": "user", "content": "Hello"}],
+    )
+    
+    # Wait for background task to complete
+    await asyncio.wait_for(background_task_done.wait(), timeout=5.0)
+    await background_task
+    
+    # Verify the response was successful
+    assert response.id == "chatcmpl-test"
+    assert response.choices[0].message.content == "Hello!"
+    
+    # The critical assertion: background task must have made significant progress
+    # If the event loop was blocked during serialization, the background task
+    # would complete much later (only after the request completes).
+    # With proper async serialization, the background task should complete
+    # most of its iterations during the request.
+    #
+    # We expect at least 50 iterations out of 100 to have completed.
+    # This threshold allows for small timing variations while still catching
+    # any significant event loop blocking.
+    assert background_task_iterations >= 50, (
+        f"Background task only completed {background_task_iterations}/100 iterations. "
+        f"Event loop may be getting blocked during JSON serialization."
+    )