diff --git a/backend/cmd/server/main.go b/backend/cmd/server/main.go
index ec8a27aa..1ea41f4b 100644
--- a/backend/cmd/server/main.go
+++ b/backend/cmd/server/main.go
@@ -265,7 +265,7 @@ func main() {
 	kratosAdminService := service.NewKratosAdminService()
 	oryAdminProvider := service.NewOryProvider()
 
-	tenantService := service.NewTenantService(tenantRepo, userRepo, ketoOutboxRepo)
+	tenantService := service.NewTenantService(tenantRepo, userRepo, userGroupRepo, ketoOutboxRepo)
 	userGroupService := service.NewUserGroupService(userGroupRepo, userRepo, tenantRepo, ketoService, ketoOutboxRepo, kratosAdminService)
 	tenantService.SetKetoService(ketoService) // Keto 주입
 
diff --git a/backend/internal/service/tenant_service.go b/backend/internal/service/tenant_service.go
index 3c7e06c5..aa71f52e 100644
--- a/backend/internal/service/tenant_service.go
+++ b/backend/internal/service/tenant_service.go
@@ -26,17 +26,19 @@ type TenantService interface {
 }
 
 type tenantService struct {
-	repo       repository.TenantRepository
-	userRepo   repository.UserRepository
-	keto       KetoService
-	outboxRepo repository.KetoOutboxRepository
+	repo          repository.TenantRepository
+	userRepo      repository.UserRepository
+	userGroupRepo repository.UserGroupRepository
+	keto          KetoService
+	outboxRepo    repository.KetoOutboxRepository
 }
 
-func NewTenantService(repo repository.TenantRepository, userRepo repository.UserRepository, outboxRepo repository.KetoOutboxRepository) TenantService {
+func NewTenantService(repo repository.TenantRepository, userRepo repository.UserRepository, userGroupRepo repository.UserGroupRepository, outboxRepo repository.KetoOutboxRepository) TenantService {
 	return &tenantService{
-		repo:       repo,
-		userRepo:   userRepo,
-		outboxRepo: outboxRepo,
+		repo:          repo,
+		userRepo:      userRepo,
+		userGroupRepo: userGroupRepo,
+		outboxRepo:    outboxRepo,
 	}
 }
 
@@ -110,6 +112,51 @@ func (s *tenantService) RegisterTenant(ctx context.Context, name, slug, tenantTy
 		return nil, err
 	}
 
+	// [New] Create Default User Groups
+	if s.userGroupRepo != nil {
+		groups := []struct {
+			Name string
+			Slug string
+		}{
+			{Name: "임직원", Slug: "members"},
+			{Name: "관리자", Slug: "admins"},
+		}
+
+		for _, g := range groups {
+			newGroup := &domain.UserGroup{
+				TenantID:    tenant.ID,
+				Name:        g.Name,
+				Slug:        g.Slug,
+				Description: tenant.Name + " " + g.Name + " 그룹",
+			}
+			if err := s.userGroupRepo.Create(ctx, newGroup); err == nil {
+				// Sync group to Keto via Outbox
+				if s.outboxRepo != nil {
+					_ = s.outboxRepo.Create(ctx, &domain.KetoOutbox{
+						Namespace: "UserGroup",
+						Object:    newGroup.ID,
+						Relation:  "tenants",
+						Subject:   "Tenant:" + tenant.ID,
+						Action:    domain.KetoOutboxActionCreate,
+					})
+					
+					// If this is the 'admins' group and we have a creatorID, add creator to this group
+					if g.Slug == "admins" && creatorID != "" {
+						_ = s.outboxRepo.Create(ctx, &domain.KetoOutbox{
+							Namespace: "UserGroup",
+							Object:    newGroup.ID,
+							Relation:  "members",
+							Subject:   "User:" + creatorID,
+							Action:    domain.KetoOutboxActionCreate,
+						})
+					}
+				}
+			} else {
+				slog.Error("Failed to create default group", "group", g.Slug, "tenant", tenant.ID, "error", err)
+			}
+		}
+	}
+
 	// [Keto] Sync hierarchy and ownership via Outbox
 	if s.outboxRepo != nil {
 		// Sync hierarchy
@@ -209,6 +256,37 @@ func (s *tenantService) ApproveTenant(ctx context.Context, id string) error {
 		return err
 	}
 
+	// [New] Create Default User Groups upon approval
+	if s.userGroupRepo != nil {
+		groups := []struct {
+			Name string
+			Slug string
+		}{
+			{Name: "임직원", Slug: "members"},
+			{Name: "관리자", Slug: "admins"},
+		}
+
+		for _, g := range groups {
+			newGroup := &domain.UserGroup{
+				TenantID:    tenant.ID,
+				Name:        g.Name,
+				Slug:        g.Slug,
+				Description: tenant.Name + " " + g.Name + " 그룹",
+			}
+			if err := s.userGroupRepo.Create(ctx, newGroup); err == nil {
+				if s.outboxRepo != nil {
+					_ = s.outboxRepo.Create(ctx, &domain.KetoOutbox{
+						Namespace: "UserGroup",
+						Object:    newGroup.ID,
+						Relation:  "tenants",
+						Subject:   "Tenant:" + tenant.ID,
+						Action:    domain.KetoOutboxActionCreate,
+					})
+				}
+			}
+		}
+	}
+
 	// [Keto] Sync relation via Outbox
 	if s.outboxRepo != nil {
 		if adminEmail, ok := tenant.Config["adminEmail"].(string); ok && adminEmail != "" {