diff --git a/app/Services/Model/Imp/MemberService.php b/app/Services/Model/Imp/MemberService.php
index ec9655e1f..60a57cff0 100644
--- a/app/Services/Model/Imp/MemberService.php
+++ b/app/Services/Model/Imp/MemberService.php
@@ -539,7 +539,7 @@ public function synchronizeGroups(Member $member, array $groups, bool $allow_rem
                             continue;
 
                         // skipping this groups bc are managed by SS side
-                        if(in_array($group->getCode(), [IGroup::FoundationMembers, IGroup::CommunityMembers, IGroup::TrackChairs, IGroup::Sponsors])){
+                        if(in_array($group->getCode(), [IGroup::FoundationMembers, IGroup::CommunityMembers, IGroup::TrackChairs])){
                             Log::debug(sprintf("MemberService::synchronizeGroups skipping group %s removal", $group->getCode()));
                             continue;
                         }
@@ -558,6 +558,15 @@ public function synchronizeGroups(Member $member, array $groups, bool $allow_rem
                             $member->removeFromGroup($group);
                         }
                     }
+                    // the IDP is the source of truth: a sponsor group that is gone takes its Sponsor_Users permission slug with it
+                    if(in_array($externalGroup->getCode(), [IGroup::Sponsors, IGroup::SponsorExternalUsers])){
+                        Log::debug(sprintf("MemberService::synchronizeGroups member %s email %s stripping Sponsor_Users permission for group %s", $member->getId(), $member->getEmail(), $externalGroup->getCode()));
+                        // same primitive the sponsor-users-api removal event uses; the Sponsor_Users rows stay
+                        // (their lifecycle belongs to the access right events), only the slug is stripped
+                        foreach ($member->getSponsorMemberships() as $sponsor) {
+                            $member->removeSponsorPermission($sponsor->getId(), $externalGroup->getCode());
+                        }
+                    }
                     Log::debug(sprintf("MemberService::synchronizeGroups member %s email %s removing from group %s", $member->getId(), $member->getEmail(), $externalGroup->getCode()));
                     $member->removeFromGroup($externalGroup);
                 }
diff --git a/tests/Unit/Services/MemberServiceSponsorGroupSyncTest.php b/tests/Unit/Services/MemberServiceSponsorGroupSyncTest.php
new file mode 100644
index 000000000..427019400
--- /dev/null
+++ b/tests/Unit/Services/MemberServiceSponsorGroupSyncTest.php
@@ -0,0 +1,177 @@
+<?php namespace Tests\Unit\Services;
+/**
+ * Copyright 2026 OpenStack Foundation
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ **/
+
+use App\Models\Foundation\Main\IGroup;
+use App\Services\Model\IMemberService;
+use Illuminate\Support\Facades\App;
+use models\main\Group;
+use Tests\InsertMemberTestData;
+use Tests\InsertSummitTestData;
+use Tests\TestCase;
+
+/**
+ * Class MemberServiceSponsorGroupSyncTest
+ *
+ * The IDP payload is the source of truth for the sponsor groups: on the full
+ * (allow_removals = true) sync a sponsor group missing from the payload is removed
+ * locally and its slug is stripped from the member's Sponsor_Users.Permissions (the
+ * Sponsor_Users rows stay: their lifecycle belongs to sponsor-users-api). The additive
+ * (per-request, token claim) sync never removes either.
+ *
+ * @package Tests\Unit\Services
+ */
+class MemberServiceSponsorGroupSyncTest extends TestCase
+{
+    use InsertSummitTestData;
+    use InsertMemberTestData;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+        self::insertMemberTestData(IGroup::Sponsors);
+        self::insertSummitTestData();
+
+        // Sponsor_Users row for sponsors[0], permission set by each test.
+        self::$sponsors[0]->addUser(self::$member);
+
+        self::$em->flush();
+        self::$em->clear();
+    }
+
+    public function tearDown(): void
+    {
+        self::clearSummitTestData();
+        self::clearMemberTestData();
+        parent::tearDown();
+    }
+
+    private function setPermissions(int $sponsor_id, int $member_id, array $slugs): void
+    {
+        self::$em->getConnection()->executeStatement(
+            'UPDATE Sponsor_Users SET Permissions = ? WHERE SponsorID = ? AND MemberID = ?',
+            [json_encode($slugs), $sponsor_id, $member_id]
+        );
+    }
+
+    /**
+     * @return array|null decoded Permissions of the row, null when the row does not exist
+     */
+    private function getPermissions(int $sponsor_id, int $member_id): ?array
+    {
+        $res = self::$em->getConnection()->fetchOne(
+            'SELECT Permissions FROM Sponsor_Users WHERE SponsorID = ? AND MemberID = ?',
+            [$sponsor_id, $member_id]
+        );
+        return $res === false ? null : json_decode($res ?? '[]', true);
+    }
+
+    private function sync(array $groups, bool $allow_removals): void
+    {
+        $member = self::$member_repository->find(self::$member->getId());
+        App::make(IMemberService::class)->synchronizeGroups($member, $groups, $allow_removals);
+        self::$em->clear();
+    }
+
+    public function testFullSyncWithoutSponsorsGroupRemovesLocalGroupAndStripsSlugFromSponsorUsersRow(): void
+    {
+        $member_id = self::$member->getId();
+        $sponsor_id = self::$sponsors[0]->getId();
+        $this->setPermissions($sponsor_id, $member_id, [IGroup::Sponsors]);
+
+        $this->sync([], true);
+
+        $member = self::$member_repository->find($member_id);
+        $this->assertFalse($member->belongsToGroup(IGroup::Sponsors), "local sponsors group must be removed");
+        // the membership itself is owned by the sponsor-users-api access right events, not by the group sync
+        $this->assertEquals([], $this->getPermissions($sponsor_id, $member_id), "slug must be stripped, row must stay");
+    }
+
+    public function testFullSyncKeepsSponsorsGroupAndSponsorUsersRowWhenPayloadStillHasIt(): void
+    {
+        $member_id = self::$member->getId();
+        $sponsor_id = self::$sponsors[0]->getId();
+        $this->setPermissions($sponsor_id, $member_id, [IGroup::Sponsors]);
+
+        $this->sync([IGroup::Sponsors], true);
+
+        $member = self::$member_repository->find($member_id);
+        $this->assertTrue($member->belongsToGroup(IGroup::Sponsors));
+        $this->assertEquals([IGroup::Sponsors], $this->getPermissions($sponsor_id, $member_id));
+    }
+
+    public function testAdditiveSyncNeverRemovesSponsorsGroupNorSponsorUsersRow(): void
+    {
+        $member_id = self::$member->getId();
+        $sponsor_id = self::$sponsors[0]->getId();
+        $this->setPermissions($sponsor_id, $member_id, [IGroup::Sponsors]);
+
+        $this->sync([], false);
+
+        $member = self::$member_repository->find($member_id);
+        $this->assertTrue($member->belongsToGroup(IGroup::Sponsors));
+        $this->assertEquals([IGroup::Sponsors], $this->getPermissions($sponsor_id, $member_id));
+    }
+
+    public function testFullSyncKeepsRowAndOtherSlugsWhenRowHoldsMoreThanTheRemovedGroup(): void
+    {
+        $member_id = self::$member->getId();
+        $sponsor_id = self::$sponsors[0]->getId();
+        $this->setPermissions($sponsor_id, $member_id, [IGroup::Sponsors, IGroup::SponsorServices]);
+
+        $this->sync([], true);
+
+        $member = self::$member_repository->find($member_id);
+        $this->assertFalse($member->belongsToGroup(IGroup::Sponsors));
+        $this->assertEquals(
+            [IGroup::SponsorServices],
+            $this->getPermissions($sponsor_id, $member_id),
+            "only the removed slug must be stripped; the row keeps the other one"
+        );
+    }
+
+    public function testFullSyncWithoutExternalUsersGroupRemovesLocalGroupAndStripsSlugFromSponsorUsersRow(): void
+    {
+        $member_id = self::$member->getId();
+        $sponsor_id = self::$sponsors[0]->getId();
+
+        $external = new Group();
+        $external->setCode(IGroup::SponsorExternalUsers);
+        $external->setTitle(IGroup::SponsorExternalUsers);
+        $external->setDescription(IGroup::SponsorExternalUsers);
+        $external->setExternal();
+        self::$em->persist($external);
+        $member = self::$member_repository->find($member_id);
+        $member->add2Group($external);
+        self::$em->flush();
+        self::$em->clear();
+
+        $this->setPermissions($sponsor_id, $member_id, [IGroup::SponsorExternalUsers]);
+
+        try {
+            // payload keeps sponsors (so the row's remaining access is judged on its own slug) but drops external users
+            $this->sync([IGroup::Sponsors], true);
+
+            $member = self::$member_repository->find($member_id);
+            $this->assertTrue($member->belongsToGroup(IGroup::Sponsors));
+            $this->assertFalse($member->belongsToGroup(IGroup::SponsorExternalUsers));
+            $this->assertEquals([], $this->getPermissions($sponsor_id, $member_id));
+        } finally {
+            $g = self::$em->getRepository(Group::class)->findOneBy(['code' => IGroup::SponsorExternalUsers]);
+            if (!is_null($g)) {
+                self::$em->remove($g);
+                self::$em->flush();
+            }
+        }
+    }
+}