diff --git a/blacksheep/server/application.py b/blacksheep/server/application.py
index 7295dd35..34ba8feb 100644
--- a/blacksheep/server/application.py
+++ b/blacksheep/server/application.py
@@ -64,7 +64,7 @@
 )
 from blacksheep.server.routing import router as default_router
 from blacksheep.server.routing import validate_router
-from blacksheep.server.websocket import WebSocket
+from blacksheep.server.websocket import WebSocket, format_reason
 from blacksheep.sessions import SessionMiddleware, SessionSerializer
 from blacksheep.settings.di import di_settings
 from blacksheep.utils import ensure_bytes, join_fragments
@@ -749,7 +749,7 @@ async def _handle_websocket(self, scope, receive, send):
             # If WebSocket connection accepted, close
             # the connection using WebSocket Internal error code.
             if ws.accepted:
-                return await ws.close(1011, reason=str(exc))
+                return await ws.close(1011, reason=format_reason(str(exc)))
 
             # Otherwise, just close the connection, the ASGI server
             # will anyway respond 403 to the client.
diff --git a/blacksheep/server/websocket.py b/blacksheep/server/websocket.py
index 7fa12a0d..b2ed40ea 100644
--- a/blacksheep/server/websocket.py
+++ b/blacksheep/server/websocket.py
@@ -6,6 +6,7 @@
 from blacksheep.server.asgi import get_full_path
 from blacksheep.settings.json import json_settings
 
+MAX_REASON_SIZE = 123
 
 class WebSocketState(Enum):
     CONNECTING = 0
@@ -180,3 +181,18 @@ async def disconnect():
 
     async def close(self, code: int = 1000, reason: Optional[str] = None) -> None:
         await self._send({"type": "websocket.close", "code": code, "reason": reason})
+
+
+def format_reason(reason: str) -> str:
+    """
+    Ensures that the close reason is no longer than the max reason size.
+    (https://developer.mozilla.org/en-US/docs/Web/API/WebSocket/close#reason)
+    """
+    reason_bytes = reason.encode()
+
+    if len(reason_bytes) <= MAX_REASON_SIZE:
+        return reason
+
+    ellipsis_ = b"..."
+    truncated_reason = reason_bytes[:MAX_REASON_SIZE - len(ellipsis_)] + ellipsis_
+    return truncated_reason.decode()
diff --git a/tests/test_websocket.py b/tests/test_websocket.py
index 4b95a387..2f113288 100644
--- a/tests/test_websocket.py
+++ b/tests/test_websocket.py
@@ -7,6 +7,7 @@
     WebSocket,
     WebSocketDisconnectError,
     WebSocketState,
+    format_reason,
 )
 from blacksheep.testing.messages import MockReceive, MockSend
 from tests.utils.application import FakeApplication
@@ -442,3 +443,20 @@ async def websocket_handler(my_ws: WebSocket):
         mock_receive,
         mock_send,
     )
+
+LONG_REASON = "WRY" * 41
+QIN = "秦"  # Qyn dynasty in Chinese, 3 bytes.
+TOO_LONG_REASON = QIN * 42
+TOO_LONG_REASON_TRUNC = TOO_LONG_REASON[:40] + "..."
+
+
+@pytest.mark.parametrize(
+    "inp,out",
+    [
+        ("Short reason", "Short reason"),
+        (LONG_REASON, LONG_REASON),
+        (TOO_LONG_REASON, TOO_LONG_REASON_TRUNC)
+    ]
+)
+def test_format_reason(inp, out):
+    assert format_reason(inp) == out