diff --git a/internal/customer/handler.go b/internal/customer/handler.go
index 410296d..52bdf7a 100644
--- a/internal/customer/handler.go
+++ b/internal/customer/handler.go
@@ -54,6 +54,7 @@ func (h *Handler) CreateCustomer(c echo.Context) error {
 	customer, err := h.service.Register(c.Request().Context(), form)
 	if err != nil {
 		if err.Error() == "customer with this email already exists" ||
+			err.Error() == "customer with this username already exists" ||
 			err.Error() == "company with this name already exists" ||
 			err.Error() == "company with this registration number already exists" ||
 			err.Error() == "company with this tax ID already exists" {
@@ -126,6 +127,7 @@ func (h *Handler) UpdateCustomer(c echo.Context) error {
 			return response.NotFound(c, "Customer not found")
 		}
 		if err.Error() == "customer with this email already exists" ||
+			err.Error() == "customer with this username already exists" ||
 			err.Error() == "company with this name already exists" ||
 			err.Error() == "company with this registration number already exists" ||
 			err.Error() == "company with this tax ID already exists" {
diff --git a/internal/customer/service.go b/internal/customer/service.go
index b644487..4cd6dba 100644
--- a/internal/customer/service.go
+++ b/internal/customer/service.go
@@ -89,6 +89,12 @@ func (s *customerService) Register(ctx context.Context, form *CreateCustomerForm
 		return nil, errors.New("customer with this email already exists")
 	}
 
+	// Check if username already exists
+	existingCustomer, _ = s.repository.GetByUsername(ctx, strings.ToLower(form.Username))
+	if existingCustomer != nil {
+		return nil, errors.New("customer with this username already exists")
+	}
+
 	// Check validator
 	if !s.validator.IsValidUsername(form.Username) {
 		return nil, errors.New("invalid username format")