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 ef8fbaf5..4b3c6c0c 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/routing/register.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/routing/register.go
@@ -49,7 +49,7 @@ const (
 var (
 	// TODO: Remove old sessions. Need to do so on a session-specific timeout.
 	sessions           = make(map[string][]authtypes.LoginType) // Sessions and completed flow stages
-	validUsernameRegex = regexp.MustCompile(`^[0-9a-zA-Z_\-./]+$`)
+	validUsernameRegex = regexp.MustCompile(`^[0-9a-z_\-./]+$`)
 )
 
 // registerRequest represents the submitted registration request.
@@ -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
 	}
@@ -278,6 +281,10 @@ func LegacyRegister(
 	if resErr != nil {
 		return *resErr
 	}
+
+	// Squash username to all lowercase letters
+	r.Username = strings.ToLower(r.Username)
+
 	if resErr = validateUserName(r.Username); resErr != nil {
 		return *resErr
 	}
@@ -478,6 +485,9 @@ func RegisterAvailable(
 ) util.JSONResponse {
 	username := req.URL.Query().Get("username")
 
+	// Squash username to all lowercase letters
+	username = strings.ToLower(username)
+
 	if err := validateUserName(username); err != nil {
 		return *err
 	}