diff --git a/src/github.com/matrix-org/dendrite/clientapi/routing/register.go b/src/github.com/matrix-org/dendrite/clientapi/routing/register.go
index ef8fbaf54..ac88e1ce1 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/routing/register.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/routing/register.go
@@ -183,6 +183,9 @@ func Register(
 		}
 	}
 
+	// Squash username to all lowercase letters
+	r.Username = strings.ToLower(r.Username)
+
 	if resErr = validateUserName(r.Username); resErr != nil {
 		return *resErr
 	}