diff --git a/src/documents/search/_backend.py b/src/documents/search/_backend.py
index 996d332e8..be9376ea4 100644
--- a/src/documents/search/_backend.py
+++ b/src/documents/search/_backend.py
@@ -31,6 +31,7 @@ from documents.search._query import parse_user_query
 from documents.search._schema import _write_sentinels
 from documents.search._schema import build_schema
 from documents.search._schema import open_or_rebuild_index
+from documents.search._schema import rebuild_in_progress
 from documents.search._schema import wipe_index
 from documents.search._tokenizer import ascii_fold
 from documents.search._tokenizer import autocomplete_tokens
@@ -1110,39 +1111,44 @@ class TantivyBackend:
                 flushing a segment, deferring merge work; they do not avoid it.
         """
         wipe_index(self._path)
-        new_index = tantivy.Index(build_schema(), path=str(self._path))
-        _write_sentinels(self._path)
-        register_tokenizers(new_index, settings.SEARCH_LANGUAGE)
+        # The marker covers the window where the empty index is already stamped
+        # as current but not yet populated, so an interrupted rebuild is retried.
+        with rebuild_in_progress(self._path):
+            new_index = tantivy.Index(build_schema(), path=str(self._path))
+            _write_sentinels(self._path)
+            register_tokenizers(new_index, settings.SEARCH_LANGUAGE)
 
-        # Point instance at the new index so _build_tantivy_doc uses it
-        old_index, old_schema = self._raw_index, self._raw_schema
-        self._raw_index = new_index
-        self._raw_schema = new_index.schema
-        # Stream documents one-by-one (so the progress bar advances per
-        # document) while fetching viewer permissions one SQL query per chunk.
-        # The stream is Sized, so iter_wrapper can still discover the total.
-        documents_stream = _DocumentViewerStream(documents, chunk_size=1000)
-        try:
-            writer = new_index.writer(heap_size=writer_heap_bytes)
-            for document, (viewer_ids, viewer_group_ids) in iter_wrapper(
-                documents_stream,
-            ):
-                doc = self._build_tantivy_doc(
-                    document,
-                    viewer_ids=viewer_ids,
-                    viewer_group_ids=viewer_group_ids,
-                )
-                writer.add_document(doc)
-            writer.commit()
-            # Wait for background merge threads to finish so all segments are
-            # fully merged and persisted before the index is considered rebuilt.
-            writer.wait_merging_threads()
-            new_index.reload()
-        except BaseException:  # pragma: no cover
-            # Restore old index on failure so the backend remains usable
-            self._raw_index = old_index
-            self._raw_schema = old_schema
-            raise
+            # Point instance at the new index so _build_tantivy_doc uses it
+            old_index, old_schema = self._raw_index, self._raw_schema
+            self._raw_index = new_index
+            self._raw_schema = new_index.schema
+            # Stream documents one-by-one (so the progress bar advances per
+            # document) while fetching viewer permissions one SQL query per
+            # chunk. The stream is Sized, so iter_wrapper can still discover
+            # the total.
+            documents_stream = _DocumentViewerStream(documents, chunk_size=1000)
+            try:
+                writer = new_index.writer(heap_size=writer_heap_bytes)
+                for document, (viewer_ids, viewer_group_ids) in iter_wrapper(
+                    documents_stream,
+                ):
+                    doc = self._build_tantivy_doc(
+                        document,
+                        viewer_ids=viewer_ids,
+                        viewer_group_ids=viewer_group_ids,
+                    )
+                    writer.add_document(doc)
+                writer.commit()
+                # Wait for background merge threads to finish so all segments
+                # are fully merged and persisted before the index is considered
+                # rebuilt.
+                writer.wait_merging_threads()
+                new_index.reload()
+            except BaseException:  # pragma: no cover
+                # Restore old index on failure so the backend remains usable
+                self._raw_index = old_index
+                self._raw_schema = old_schema
+                raise
 
 
 def chunked(iterable, size):
diff --git a/src/documents/search/_schema.py b/src/documents/search/_schema.py
index 2539d12fe..62130c246 100644
--- a/src/documents/search/_schema.py
+++ b/src/documents/search/_schema.py
@@ -4,6 +4,7 @@ import hashlib
 import json
 import logging
 import shutil
+from contextlib import contextmanager
 from typing import TYPE_CHECKING
 from typing import Final
 from typing import NamedTuple
@@ -16,6 +17,7 @@ from whoosh_compat import FieldKind
 from documents.search._fields import PUBLIC_FIELDS
 
 if TYPE_CHECKING:
+    from collections.abc import Iterator
     from pathlib import Path
 
 logger = logging.getLogger("paperless.search")
@@ -28,6 +30,11 @@ logger = logging.getLogger("paperless.search")
 # v3 - barcodes JSON field for stored barcode contents
 SCHEMA_VERSION: Final[int] = 3
 
+# Present in the index directory from the moment a full rebuild starts until it
+# finishes. If a rebuild is interrupted it is left behind, so the half-built
+# index is not mistaken for a complete one.
+REBUILD_MARKER: Final[str] = ".rebuilding"
+
 
 class FieldDescriptor(NamedTuple):
     """One tantivy field, in declaration order.
@@ -255,9 +262,9 @@ def needs_rebuild(index_dir: Path) -> bool:
     """
     Check if the search index needs rebuilding.
 
-    Reads .index_settings.json to compare the stored schema version, search
-    language and schema fingerprint against the current configuration. Returns
-    True if the file is missing, unparsable, or any value mismatches.
+    True if a previous full rebuild never finished (the rebuild marker is still
+    present), or if the index's stamped settings no longer match the current
+    configuration. See _settings_mismatch().
 
     Args:
         index_dir: Path to the search index directory
@@ -265,6 +272,40 @@ def needs_rebuild(index_dir: Path) -> bool:
     Returns:
         True if the index needs rebuilding, False if it's up to date
     """
+    if (index_dir / REBUILD_MARKER).exists():
+        logger.warning("Previous search index rebuild did not finish - rebuilding.")
+        return True
+    return _settings_mismatch(index_dir)
+
+
+@contextmanager
+def rebuild_in_progress(index_dir: Path) -> Iterator[None]:
+    """
+    Flag the index as incomplete for the duration of a full rebuild.
+
+    The marker is cleared only if the block exits cleanly. There is deliberately
+    no try/finally: an exception must leave the marker behind so the next
+    needs_rebuild() check retries the rebuild.
+    """
+    marker = index_dir / REBUILD_MARKER
+    marker.touch()
+    yield
+    marker.unlink(missing_ok=True)
+
+
+def _settings_mismatch(index_dir: Path) -> bool:
+    """
+    Check the stamped settings against the current configuration.
+
+    Reads .index_settings.json to compare the stored schema version, search
+    language and schema fingerprint. Returns True if the file is missing,
+    unparsable, or any value mismatches.
+
+    This deliberately ignores the rebuild marker: open_or_rebuild_index() uses it
+    so that a process opening the index while another process is mid-rebuild
+    (or after one died) does not wipe the partial index out from under it.
+    Repopulating is the job of ``document_index reindex``.
+    """
     settings_file = index_dir / ".index_settings.json"
     if not settings_file.exists():
         return True
@@ -333,7 +374,7 @@ def open_or_rebuild_index(index_dir: Path | None = None) -> tantivy.Index:
         index_dir = cast("Path", settings.INDEX_DIR)
     if not index_dir.exists():
         return tantivy.Index(build_schema())
-    if needs_rebuild(index_dir):
+    if _settings_mismatch(index_dir):
         wipe_index(index_dir)
         idx = tantivy.Index(build_schema(), path=str(index_dir))
         _write_sentinels(index_dir)
diff --git a/src/documents/tests/search/test_backend.py b/src/documents/tests/search/test_backend.py
index d215e3ca7..223907e19 100644
--- a/src/documents/tests/search/test_backend.py
+++ b/src/documents/tests/search/test_backend.py
@@ -16,7 +16,10 @@ from documents.search._backend import TantivyBackend
 from documents.search._backend import WriteBatch
 from documents.search._backend import get_backend
 from documents.search._backend import reset_backend
+from documents.search._schema import REBUILD_MARKER
+from documents.search._schema import needs_rebuild
 from documents.signals.handlers import add_to_index
+from paperless_testing.dirs import PaperlessDirs
 from paperless_testing.factories import CorrespondentFactory
 from paperless_testing.factories import DocumentFactory
 from paperless_testing.factories import DocumentTypeFactory
@@ -823,6 +826,53 @@ class TestRebuild:
         backend.rebuild(Document.objects.all(), iter_wrapper=wrapper)
         assert 30 in seen
 
+    def test_successful_rebuild_leaves_index_up_to_date(
+        self,
+        backend: TantivyBackend,
+        paperless_dirs: PaperlessDirs,
+    ) -> None:
+        """
+        GIVEN:
+            - A backend and one document
+        WHEN:
+            - rebuild() completes
+        THEN:
+            - needs_rebuild() is False and no rebuild marker remains
+        """
+        DocumentFactory.create()
+
+        backend.rebuild(Document.objects.all())
+
+        assert needs_rebuild(paperless_dirs.index_dir) is False
+        assert not (paperless_dirs.index_dir / REBUILD_MARKER).exists()
+
+    def test_interrupted_rebuild_is_retried(
+        self,
+        backend: TantivyBackend,
+        paperless_dirs: PaperlessDirs,
+    ) -> None:
+        """
+        GIVEN:
+            - A rebuild that dies while indexing documents (e.g. the database
+              connection is lost)
+        WHEN:
+            - needs_rebuild() is checked afterwards
+        THEN:
+            - It is True, even though the empty index was already stamped with
+              current settings, so the next start rebuilds instead of reporting
+              the index as up to date
+        """
+        DocumentFactory.create()
+
+        def die(pairs):
+            raise RuntimeError("terminating connection due to administrator command")
+            yield  # pragma: no cover
+
+        with pytest.raises(RuntimeError):
+            backend.rebuild(Document.objects.all(), iter_wrapper=die)
+
+        assert needs_rebuild(paperless_dirs.index_dir) is True
+
     def test_includes_group_granted_viewers(self, backend: TantivyBackend) -> None:
         """Rebuild must index viewer ids for group-only grants, not just direct ones.