diff --git a/CHANGES/3511.bugfix b/CHANGES/3511.bugfix
new file mode 100644
index 00000000000..0eef1d06cc1
--- /dev/null
+++ b/CHANGES/3511.bugfix
@@ -0,0 +1 @@
+Fix ``aiohttp.ClientTimeout`` type annotations to accept ``None`` for fields
diff --git a/aiohttp/client.py b/aiohttp/client.py
index ef31df8f013..163bb107e45 100644
--- a/aiohttp/client.py
+++ b/aiohttp/client.py
@@ -129,10 +129,10 @@
 
 @attr.s(frozen=True, slots=True)
 class ClientTimeout:
-    total = attr.ib(type=float, default=None)
-    connect = attr.ib(type=float, default=None)
-    sock_read = attr.ib(type=float, default=None)
-    sock_connect = attr.ib(type=float, default=None)
+    total = attr.ib(type=Optional[float], default=None)
+    connect = attr.ib(type=Optional[float], default=None)
+    sock_read = attr.ib(type=Optional[float], default=None)
+    sock_connect = attr.ib(type=Optional[float], default=None)
 
     # pool_queue_timeout = attr.ib(type=float, default=None)
     # dns_resolution_timeout = attr.ib(type=float, default=None)