diff --git a/connector/connector.go b/connector/connector.go
index b1e069c3..1e554789 100644
--- a/connector/connector.go
+++ b/connector/connector.go
@@ -3,9 +3,22 @@ package connector
 
 import (
 	"context"
+	"fmt"
 	"net/http"
 )
 
+// UserNotInRequiredGroupsError is returned by a connector when a user
+// successfully authenticates but is not a member of any of the required groups.
+// The server will respond with HTTP 403 Forbidden instead of 500.
+type UserNotInRequiredGroupsError struct {
+	UserID string
+	Groups []string
+}
+
+func (e *UserNotInRequiredGroupsError) Error() string {
+	return fmt.Sprintf("user %q is not in any of the required groups %v", e.UserID, e.Groups)
+}
+
 // Connector is a mechanism for federating login to a remote identity service.
 //
 // Implementations are expected to implement either the PasswordConnector or
diff --git a/connector/microsoft/microsoft.go b/connector/microsoft/microsoft.go
index 1db16942..ca6e025d 100644
--- a/connector/microsoft/microsoft.go
+++ b/connector/microsoft/microsoft.go
@@ -227,7 +227,7 @@ func (c *microsoftConnector) HandleCallback(s connector.Scopes, connData []byte,
 	if c.groupsRequired(s.Groups) {
 		groups, err := c.getGroups(ctx, client, user.ID)
 		if err != nil {
-			return identity, fmt.Errorf("microsoft: get groups: %v", err)
+			return identity, fmt.Errorf("microsoft: get groups: %w", err)
 		}
 		identity.Groups = groups
 	}
@@ -318,7 +318,7 @@ func (c *microsoftConnector) Refresh(ctx context.Context, s connector.Scopes, id
 	if c.groupsRequired(s.Groups) {
 		groups, err := c.getGroups(ctx, client, user.ID)
 		if err != nil {
-			return identity, fmt.Errorf("microsoft: get groups: %v", err)
+			return identity, fmt.Errorf("microsoft: get groups: %w", err)
 		}
 		identity.Groups = groups
 	}
@@ -404,7 +404,7 @@ func (c *microsoftConnector) getGroups(ctx context.Context, client *http.Client,
 	// ensure that the user is in at least one required group
 	filteredGroups := groups_pkg.Filter(userGroups, c.groups)
 	if len(c.groups) > 0 && len(filteredGroups) == 0 {
-		return nil, fmt.Errorf("microsoft: user %v not in any of the required groups", userID)
+		return nil, &connector.UserNotInRequiredGroupsError{UserID: userID, Groups: c.groups}
 	} else if c.useGroupsAsWhitelist {
 		return filteredGroups, nil
 	}
diff --git a/connector/microsoft/microsoft_test.go b/connector/microsoft/microsoft_test.go
index 1fa2f3dc..e7443bf4 100644
--- a/connector/microsoft/microsoft_test.go
+++ b/connector/microsoft/microsoft_test.go
@@ -2,6 +2,7 @@ package microsoft
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"net/http"
 	"net/http/httptest"
@@ -119,6 +120,39 @@ func TestUserGroupsFromGraphAPI(t *testing.T) {
 	expectEquals(t, identity.Groups, []string{"a", "b"})
 }
 
+func TestUserNotInRequiredGroupFromGraphAPI(t *testing.T) {
+	s := newTestServer(map[string]testResponse{
+		"/v1.0/me?$select=id,displayName,userPrincipalName": {
+			data: user{ID: "user-id-123", Name: "Jane Doe", Email: "jane.doe@example.com"},
+		},
+		// The user is a member of groups "c" and "d", but the connector only
+		// allows group "a" — so the user should be denied.
+		"/v1.0/me/getMemberGroups": {data: map[string]interface{}{
+			"value": []string{"c", "d"},
+		}},
+		"/" + tenant + "/oauth2/v2.0/token": dummyToken,
+	})
+	defer s.Close()
+
+	req, _ := http.NewRequest("GET", s.URL, nil)
+
+	c := microsoftConnector{
+		apiURL:   s.URL,
+		graphURL: s.URL,
+		tenant:   tenant,
+		groups:   []string{"a"},
+	}
+	_, err := c.HandleCallback(connector.Scopes{Groups: true}, req)
+	if err == nil {
+		t.Fatal("expected error when user is not in any required group, got nil")
+	}
+
+	var groupsErr *connector.UserNotInRequiredGroupsError
+	if !errors.As(err, &groupsErr) {
+		t.Errorf("expected *connector.UserNotInRequiredGroupsError, got %T: %v", err, err)
+	}
+}
+
 func newTestServer(responses map[string]testResponse) *httptest.Server {
 	s := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		response, found := responses[r.RequestURI]
diff --git a/server/errors.go b/server/errors.go
index c0b9d425..ec9146e0 100644
--- a/server/errors.go
+++ b/server/errors.go
@@ -23,4 +23,8 @@ const (
 
 	// ErrMsgMethodNotAllowed is shown when an unsupported HTTP method is used.
 	ErrMsgMethodNotAllowed = "Method not allowed."
+
+	// ErrMsgNotInRequiredGroups is shown when a user authenticates successfully
+	// but is not a member of any of the groups required by the connector.
+	ErrMsgNotInRequiredGroups = "You are not a member of any of the required groups to authenticate."
 )
diff --git a/server/handlers.go b/server/handlers.go
index b8e11ce4..62f1650c 100644
--- a/server/handlers.go
+++ b/server/handlers.go
@@ -7,6 +7,7 @@ import (
 	"crypto/subtle"
 	"encoding/base64"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"html/template"
 	"net/http"
@@ -499,7 +500,12 @@ func (s *Server) handleConnectorCallback(w http.ResponseWriter, r *http.Request)
 
 	if err != nil {
 		s.logger.ErrorContext(r.Context(), "failed to authenticate", "err", err)
-		s.renderError(r, w, http.StatusInternalServerError, ErrMsgAuthenticationFailed)
+		var groupsErr *connector.UserNotInRequiredGroupsError
+		if errors.As(err, &groupsErr) {
+			s.renderError(r, w, http.StatusForbidden, ErrMsgNotInRequiredGroups)
+		} else {
+			s.renderError(r, w, http.StatusInternalServerError, ErrMsgAuthenticationFailed)
+		}
 		return
 	}