diff --git a/backend/src/main/java/org/raddatz/familienarchiv/document/DocumentService.java b/backend/src/main/java/org/raddatz/familienarchiv/document/DocumentService.java
index 4106332a..1cef9593 100644
--- a/backend/src/main/java/org/raddatz/familienarchiv/document/DocumentService.java
+++ b/backend/src/main/java/org/raddatz/familienarchiv/document/DocumentService.java
@@ -797,10 +797,17 @@ public class DocumentService {
         return transcriptionBlockQueryService.getCompletionStats(docIds);
     }
 
-    private Sort resolveSort(DocumentSort sort, String dir) {
+    Sort resolveSort(DocumentSort sort, String dir) {
         Sort.Direction direction = "ASC".equalsIgnoreCase(dir) ? Sort.Direction.ASC : Sort.Direction.DESC;
         if (sort == null || sort == DocumentSort.DATE || sort == DocumentSort.RELEVANCE) {
-            return Sort.by(direction, "documentDate");
+            // Undated documents (null documentDate) must order last regardless of
+            // direction — Postgres puts NULLs FIRST on ASC by default, which would
+            // surface the undated pile at the top with no explanation (issue #668).
+            // The createdAt tiebreaker gives a stable total order when every row is
+            // null-dated (the "Nur undatierte" filter), so pagination is deterministic.
+            return Sort.by(
+                    new Sort.Order(direction, "documentDate").nullsLast(),
+                    Sort.Order.asc("createdAt"));
         }
         // SENDER and RECEIVER are sorted in-memory before this method is called
         return switch (sort) {
diff --git a/backend/src/test/java/org/raddatz/familienarchiv/document/DocumentServiceTest.java b/backend/src/test/java/org/raddatz/familienarchiv/document/DocumentServiceTest.java
index 658d4c31..32a4b3b9 100644
--- a/backend/src/test/java/org/raddatz/familienarchiv/document/DocumentServiceTest.java
+++ b/backend/src/test/java/org/raddatz/familienarchiv/document/DocumentServiceTest.java
@@ -1450,6 +1450,42 @@ class DocumentServiceTest {
         assertThat(result.items()).hasSize(1); // only the slice is enriched
     }
 
+    @Test
+    void searchDocuments_dateSort_DESC_ordersUndatedLast() {
+        ArgumentCaptor<Pageable> captor = ArgumentCaptor.forClass(Pageable.class);
+        when(documentRepository.findAll(any(org.springframework.data.jpa.domain.Specification.class), any(Pageable.class)))
+                .thenReturn(new PageImpl<>(List.of()));
+
+        documentService.searchDocuments(null, null, null, null, null, null, null, null,
+                DocumentSort.DATE, "DESC", null,
+                org.springframework.data.domain.PageRequest.of(0, 5));
+
+        verify(documentRepository).findAll(any(org.springframework.data.jpa.domain.Specification.class), captor.capture());
+        Sort.Order dateOrder = captor.getValue().getSort().getOrderFor("documentDate");
+        assertThat(dateOrder).isNotNull();
+        assertThat(dateOrder.getDirection()).isEqualTo(Sort.Direction.DESC);
+        assertThat(dateOrder.getNullHandling()).isEqualTo(Sort.NullHandling.NULLS_LAST);
+    }
+
+    @Test
+    void searchDocuments_dateSort_ASC_ordersUndatedLast() {
+        // The ASC bug: Postgres puts NULLs FIRST on ascending sort without explicit
+        // NULLS LAST, surfacing undated documents at the top. This is the red.
+        ArgumentCaptor<Pageable> captor = ArgumentCaptor.forClass(Pageable.class);
+        when(documentRepository.findAll(any(org.springframework.data.jpa.domain.Specification.class), any(Pageable.class)))
+                .thenReturn(new PageImpl<>(List.of()));
+
+        documentService.searchDocuments(null, null, null, null, null, null, null, null,
+                DocumentSort.DATE, "ASC", null,
+                org.springframework.data.domain.PageRequest.of(0, 5));
+
+        verify(documentRepository).findAll(any(org.springframework.data.jpa.domain.Specification.class), captor.capture());
+        Sort.Order dateOrder = captor.getValue().getSort().getOrderFor("documentDate");
+        assertThat(dateOrder).isNotNull();
+        assertThat(dateOrder.getDirection()).isEqualTo(Sort.Direction.ASC);
+        assertThat(dateOrder.getNullHandling()).isEqualTo(Sort.NullHandling.NULLS_LAST);
+    }
+
     @Test
     void searchDocuments_UPDATED_AT_sort_resolves_to_updatedAt_field() {
         ArgumentCaptor<Pageable> captor = ArgumentCaptor.forClass(Pageable.class);