diff --git a/aiohttp/client.py b/aiohttp/client.py
index 84b06defb6e..1a912ecfc0e 100644
--- a/aiohttp/client.py
+++ b/aiohttp/client.py
@@ -3,6 +3,7 @@
 import asyncio
 import base64
 import hashlib
+import json
 import os
 import sys
 import traceback
@@ -12,7 +13,7 @@
 from yarl import URL
 
 from . import connector as connector_mod
-from . import client_exceptions, client_reqrep, hdrs, http
+from . import client_exceptions, client_reqrep, hdrs, http, payload
 from .client_exceptions import *  # noqa
 from .client_exceptions import (ClientError, ClientOSError,
                                 ClientResponseError, ServerTimeoutError,
@@ -45,8 +46,8 @@ class ClientSession:
 
     def __init__(self, *, connector=None, loop=None, cookies=None,
                  headers=None, skip_auto_headers=None,
-                 auth=None, request_class=ClientRequest,
-                 response_class=ClientResponse,
+                 auth=None, json_serialize=json.dumps,
+                 request_class=ClientRequest, response_class=ClientResponse,
                  ws_response_class=ClientWebSocketResponse,
                  version=http.HttpVersion11,
                  cookie_jar=None, connector_owner=True, raise_for_status=False,
@@ -93,6 +94,7 @@ def __init__(self, *, connector=None, loop=None, cookies=None,
         self._connector_owner = connector_owner
         self._default_auth = auth
         self._version = version
+        self._json_serialize = json_serialize
         self._read_timeout = read_timeout
         self._conn_timeout = conn_timeout
         self._raise_for_status = raise_for_status
@@ -164,6 +166,8 @@ def _request(self, method, url, *,
         if data is not None and json is not None:
             raise ValueError(
                 'data and json parameters can not be used at the same time')
+        elif json is not None:
+            data = payload.JsonPayload(json)
 
         if not isinstance(chunked, bool) and chunked is not None:
             warnings.warn(
@@ -208,7 +212,7 @@ def _request(self, method, url, *,
 
                     req = self._request_class(
                         method, url, params=params, headers=headers,
-                        skip_auto_headers=skip_headers, data=data, json=json,
+                        skip_auto_headers=skip_headers, data=data,
                         cookies=cookies, auth=auth, version=version,
                         compress=compress, chunked=chunked,
                         expect100=expect100, loop=self._loop,
diff --git a/aiohttp/client_reqrep.py b/aiohttp/client_reqrep.py
index a67d49a8a56..423347fa531 100644
--- a/aiohttp/client_reqrep.py
+++ b/aiohttp/client_reqrep.py
@@ -54,7 +54,7 @@ class ClientRequest:
 
     def __init__(self, method, url, *,
                  params=None, headers=None, skip_auto_headers=frozenset(),
-                 data=None, json=None, cookies=None,
+                 data=None, cookies=None,
                  auth=None, version=http.HttpVersion11, compress=None,
                  chunked=None, expect100=False,
                  loop=None, response_class=None,
@@ -84,9 +84,6 @@ def __init__(self, method, url, *,
         if loop.get_debug():
             self._source_traceback = traceback.extract_stack(sys._getframe(1))
 
-        if json is not None:
-            data = payload.JsonPayload(json)
-
         self.update_version(version)
         self.update_host(url)
         self.update_headers(headers)
diff --git a/aiohttp/payload.py b/aiohttp/payload.py
index 334dfb06905..ce40dd43668 100644
--- a/aiohttp/payload.py
+++ b/aiohttp/payload.py
@@ -289,10 +289,10 @@ class JsonPayload(BytesPayload):
 
     def __init__(self, value,
                  encoding='utf-8', content_type='application/json',
-                 *args, **kwargs):
+                 dumps=json.dumps, *args, **kwargs):
 
         super().__init__(
-            json.dumps(value).encode(encoding),
+            dumps(value).encode(encoding),
             content_type=content_type, encoding=encoding, *args, **kwargs)
 
 
diff --git a/docs/client_reference.rst b/docs/client_reference.rst
index b0cd3b6e2fa..0ddb0569aff 100644
--- a/docs/client_reference.rst
+++ b/docs/client_reference.rst
@@ -43,7 +43,8 @@ The client session supports the context manager protocol for self closing.
 
 .. class:: ClientSession(*, connector=None, loop=None, cookies=None, \
                          headers=None, skip_auto_headers=None, \
-                         auth=None, version=aiohttp.HttpVersion11, \
+                         auth=None, json_serialize=json.dumps, \
+                         version=aiohttp.HttpVersion11, \
                          cookie_jar=None, read_timeout=None, conn_timeout=None, \
                          raise_for_status=False)
 
@@ -100,6 +101,8 @@ The client session supports the context manager protocol for self closing.
 
       .. versionadded:: 0.22
 
+   :param callable json_serialize: Json serializer function. (:func:`json.dumps` by default)
+
    :param bool raise_for_status: Automatically call `raise_for_status()` for each response.
       (default is False)
 
diff --git a/docs/web_reference.rst b/docs/web_reference.rst
index 1a2d8866ecc..f22ae794142 100644
--- a/docs/web_reference.rst
+++ b/docs/web_reference.rst
@@ -241,7 +241,7 @@ and :ref:`aiohttp-web-signals` handlers.
 
       Clone itself with replacement some attributes.
 
-      Creates and returns a new instance of Request object. If no parameters
+               Creates and returns a new instance of Request object. If no parameters
       are given, an exact copy is returned. If a parameter is not passed, it
       will reuse the one from the current request object.