diff --git a/backend/src/main/resources/db/migration/V55__add_spouse_symmetric_index.sql b/backend/src/main/resources/db/migration/V55__add_spouse_symmetric_index.sql
new file mode 100644
index 00000000..cee18767
--- /dev/null
+++ b/backend/src/main/resources/db/migration/V55__add_spouse_symmetric_index.sql
@@ -0,0 +1,6 @@
+-- Symmetric SPOUSE_OF: enforce only one row per unordered pair, mirroring the
+-- SIBLING_OF index added in V54.
+CREATE UNIQUE INDEX unique_spouse_pair ON person_relationships (
+    LEAST(person_id, related_person_id),
+    GREATEST(person_id, related_person_id)
+) WHERE relation_type = 'SPOUSE_OF';
diff --git a/backend/src/test/java/org/raddatz/familienarchiv/relationship/RelationshipServiceIntegrationTest.java b/backend/src/test/java/org/raddatz/familienarchiv/relationship/RelationshipServiceIntegrationTest.java
index 8654fa1d..003922ab 100644
--- a/backend/src/test/java/org/raddatz/familienarchiv/relationship/RelationshipServiceIntegrationTest.java
+++ b/backend/src/test/java/org/raddatz/familienarchiv/relationship/RelationshipServiceIntegrationTest.java
@@ -117,6 +117,20 @@ class RelationshipServiceIntegrationTest {
         assertThat(relationshipRepository.findById(created.id())).isPresent();
     }
 
+    @Test
+    void addRelationship_throws_409_when_reverse_SPOUSE_OF_pair_already_exists() {
+        // V55 enforces symmetric uniqueness for SPOUSE_OF. Inserting (alice, bob, SPOUSE_OF)
+        // and then (bob, alice, SPOUSE_OF) must be rejected, just like reverse SIBLING_OF.
+        relationshipService.addRelationship(alice.getId(),
+                new CreateRelationshipRequest(bob.getId(), "SPOUSE_OF", null, null, null));
+
+        var reverse = new CreateRelationshipRequest(alice.getId(), "SPOUSE_OF", null, null, null);
+        assertThatThrownBy(() -> relationshipService.addRelationship(bob.getId(), reverse))
+                .isInstanceOf(DomainException.class)
+                .extracting("code")
+                .isEqualTo(ErrorCode.DUPLICATE_RELATIONSHIP);
+    }
+
     @Test
     void deleteRelationship_succeeds_for_symmetric_type_from_either_side() {
         // alice SPOUSE_OF bob. Bob deletes from his side.