diff --git a/internal/oidc/callback/callback_handler.go b/internal/oidc/callback/callback_handler.go
index 97731aec..c63a544b 100644
--- a/internal/oidc/callback/callback_handler.go
+++ b/internal/oidc/callback/callback_handler.go
@@ -254,7 +254,7 @@ func getSubjectAndUsernameFromUpstreamIDToken(
 func getGroupsFromUpstreamIDToken(
 	upstreamIDPConfig provider.UpstreamOIDCIdentityProviderI,
 	idTokenClaims map[string]interface{},
-) ([]string, error) {
+) (interface{}, error) {
 	groupsClaim := upstreamIDPConfig.GetGroupsClaim()
 	if groupsClaim == "" {
 		return nil, nil
@@ -271,8 +271,9 @@ func getGroupsFromUpstreamIDToken(
 		return nil, httperr.New(http.StatusUnprocessableEntity, "no groups claim in upstream ID token")
 	}
 
-	groups, ok := groupsAsInterface.([]string)
-	if !ok {
+	groupsAsArray, okAsArray := groupsAsInterface.([]string)
+	groupsAsString, okAsString := groupsAsInterface.(string)
+	if !okAsArray && !okAsString {
 		plog.Warning(
 			"groups claim in upstream ID token has invalid format",
 			"upstreamName", upstreamIDPConfig.GetName(),
@@ -282,10 +283,13 @@ func getGroupsFromUpstreamIDToken(
 		return nil, httperr.New(http.StatusUnprocessableEntity, "groups claim in upstream ID token has invalid format")
 	}
 
-	return groups, nil
+	if okAsArray {
+		return groupsAsArray, nil
+	}
+	return groupsAsString, nil
 }
 
-func makeDownstreamSession(subject string, username string, groups []string) *openid.DefaultSession {
+func makeDownstreamSession(subject string, username string, groups interface{}) *openid.DefaultSession {
 	now := time.Now().UTC()
 	openIDSession := &openid.DefaultSession{
 		Claims: &jwt.IDTokenClaims{
diff --git a/internal/oidc/callback/callback_handler_test.go b/internal/oidc/callback/callback_handler_test.go
index eb7e4eea..2bb1411a 100644
--- a/internal/oidc/callback/callback_handler_test.go
+++ b/internal/oidc/callback/callback_handler_test.go
@@ -134,7 +134,7 @@ func TestCallbackEndpoint(t *testing.T) {
 		wantDownstreamGrantedScopes       []string
 		wantDownstreamIDTokenSubject      string
 		wantDownstreamIDTokenUsername     string
-		wantDownstreamIDTokenGroups       []string
+		wantDownstreamIDTokenGroups       interface{}
 		wantDownstreamRequestedScopes     []string
 		wantDownstreamNonce               string
 		wantDownstreamPKCEChallenge       string
@@ -199,6 +199,25 @@ func TestCallbackEndpoint(t *testing.T) {
 			wantDownstreamPKCEChallengeMethod: downstreamPKCEChallengeMethod,
 			wantExchangeAndValidateTokensCall: happyExchangeAndValidateTokensArgs,
 		},
+		{
+			name:                              "upstream IDP's configured groups claim in the ID token has a non-array value",
+			idp:                               happyUpstream().WithIDTokenClaim(upstreamGroupsClaim, "notAnArrayGroup1 notAnArrayGroup2").Build(),
+			method:                            http.MethodGet,
+			path:                              newRequestPath().WithState(happyState).String(),
+			csrfCookie:                        happyCSRFCookie,
+			wantStatus:                        http.StatusFound,
+			wantRedirectLocationRegexp:        happyDownstreamRedirectLocationRegexp,
+			wantBody:                          "",
+			wantDownstreamIDTokenSubject:      upstreamIssuer + "?sub=" + upstreamSubject,
+			wantDownstreamIDTokenUsername:     upstreamUsername,
+			wantDownstreamIDTokenGroups:       "notAnArrayGroup1 notAnArrayGroup2",
+			wantDownstreamRequestedScopes:     happyDownstreamScopesRequested,
+			wantDownstreamGrantedScopes:       happyDownstreamScopesGranted,
+			wantDownstreamNonce:               downstreamNonce,
+			wantDownstreamPKCEChallenge:       downstreamPKCEChallenge,
+			wantDownstreamPKCEChallengeMethod: downstreamPKCEChallengeMethod,
+			wantExchangeAndValidateTokensCall: happyExchangeAndValidateTokensArgs,
+		},
 
 		// Pre-upstream-exchange verification
 		{
@@ -682,8 +701,8 @@ func happyUpstream() *upstreamOIDCIdentityProviderBuilder {
 	}
 }
 
-func (u *upstreamOIDCIdentityProviderBuilder) WithUsernameClaim(claim string) *upstreamOIDCIdentityProviderBuilder {
-	u.usernameClaim = claim
+func (u *upstreamOIDCIdentityProviderBuilder) WithUsernameClaim(value string) *upstreamOIDCIdentityProviderBuilder {
+	u.usernameClaim = value
 	return u
 }
 
@@ -750,7 +769,7 @@ func validateAuthcodeStorage(
 	wantDownstreamGrantedScopes []string,
 	wantDownstreamIDTokenSubject string,
 	wantDownstreamIDTokenUsername string,
-	wantDownstreamIDTokenGroups []string,
+	wantDownstreamIDTokenGroups interface{},
 	wantDownstreamRequestedScopes []string,
 ) (*fosite.Request, *openid.DefaultSession) {
 	t.Helper()
@@ -789,9 +808,19 @@ func validateAuthcodeStorage(
 	// Check the user's identity, which are put into the downstream ID token's subject, username and groups claims.
 	require.Equal(t, wantDownstreamIDTokenSubject, actualClaims.Subject)
 	require.Equal(t, wantDownstreamIDTokenUsername, actualClaims.Extra["username"])
-	if wantDownstreamIDTokenGroups != nil {
+	if wantDownstreamIDTokenGroups != nil { //nolint:nestif // there are some nested if's here but its probably fine for a test
 		require.Len(t, actualClaims.Extra, 2)
-		require.ElementsMatch(t, wantDownstreamIDTokenGroups, actualClaims.Extra["groups"])
+		wantArray, ok := wantDownstreamIDTokenGroups.([]string)
+		if ok {
+			require.ElementsMatch(t, wantArray, actualClaims.Extra["groups"])
+		} else {
+			wantString, ok := wantDownstreamIDTokenGroups.(string)
+			if ok {
+				require.Equal(t, wantString, actualClaims.Extra["groups"])
+			} else {
+				require.Fail(t, "wantDownstreamIDTokenGroups should be of type: either []string or string")
+			}
+		}
 	} else {
 		require.Len(t, actualClaims.Extra, 1)
 		require.NotContains(t, actualClaims.Extra, "groups")