diff --git a/starlette/testclient.py b/starlette/testclient.py
index bdae83bf0..a66b901d6 100644
--- a/starlette/testclient.py
+++ b/starlette/testclient.py
@@ -145,7 +145,7 @@ def send_bytes(self, data: bytes) -> None:
 
     def send_json(self, data: typing.Any, mode: str = "text") -> None:
         assert mode in ["text", "binary"]
-        text = json.dumps(data)
+        text = json.dumps(data, separators=(",", ":"))
         if mode == "text":
             self.send({"type": "websocket.receive", "text": text})
         else:
diff --git a/starlette/websockets.py b/starlette/websockets.py
index afcbde7fc..e89579235 100644
--- a/starlette/websockets.py
+++ b/starlette/websockets.py
@@ -168,7 +168,7 @@ async def send_bytes(self, data: bytes) -> None:
     async def send_json(self, data: typing.Any, mode: str = "text") -> None:
         if mode not in {"text", "binary"}:
             raise RuntimeError('The "mode" argument should be "text" or "binary".')
-        text = json.dumps(data)
+        text = json.dumps(data, separators=(",", ":"))
         if mode == "text":
             await self.send({"type": "websocket.send", "text": text})
         else: