diff --git a/src/documents/conditionals.py b/src/documents/conditionals.py
index fa10ff58a..82114e8d6 100644
--- a/src/documents/conditionals.py
+++ b/src/documents/conditionals.py
@@ -117,6 +117,17 @@ def preview_last_modified(request, pk: int) -> datetime | None:
     return doc.modified
 
 
+def thumbnail_etag(request: Any, pk: int) -> str | None:
+    """
+    Thumbnails are version-dependent, so use the effective document checksum as
+    the ETag to invalidate cache when the latest version changes.
+    """
+    doc = resolve_effective_document_by_pk(pk, request).document
+    if doc is None:
+        return None
+    return doc.checksum
+
+
 def thumbnail_last_modified(request: Any, pk: int) -> datetime | None:
     """
     Returns the filesystem last modified either from cache or from filesystem.
diff --git a/src/documents/tests/test_api_document_versions.py b/src/documents/tests/test_api_document_versions.py
index 848c6ec21..a81b9d545 100644
--- a/src/documents/tests/test_api_document_versions.py
+++ b/src/documents/tests/test_api_document_versions.py
@@ -464,6 +464,40 @@ class TestDocumentVersioningApi(DirectoriesMixin, APITestCase):
         self.assertEqual(resp.status_code, status.HTTP_200_OK)
         self.assertEqual(read_streaming_response(resp), b"thumb")
 
+    def test_thumb_etag_changes_when_latest_version_is_deleted(self) -> None:
+        root = self._create_pdf(title="root", checksum="root")
+        v1 = self._create_pdf(
+            title="v1",
+            checksum="v1",
+            root_document=root,
+        )
+        v2 = self._create_pdf(
+            title="v2",
+            checksum="v2",
+            root_document=root,
+        )
+        self._write_file(v1.thumbnail_path, b"thumb-v1")
+        self._write_file(v2.thumbnail_path, b"thumb-v2")
+
+        resp = self.client.get(f"/api/documents/{root.id}/thumb/")
+        self.assertEqual(resp.status_code, status.HTTP_200_OK)
+        self.assertEqual(read_streaming_response(resp), b"thumb-v2")
+        self.assertEqual(resp.headers["ETag"], '"v2"')
+
+        with mock.patch("documents.search.get_backend"):
+            delete_resp = self.client.delete(
+                f"/api/documents/{root.id}/versions/{v2.id}/",
+            )
+        self.assertEqual(delete_resp.status_code, status.HTTP_200_OK)
+
+        resp = self.client.get(
+            f"/api/documents/{root.id}/thumb/",
+            HTTP_IF_NONE_MATCH='"v2"',
+        )
+        self.assertEqual(resp.status_code, status.HTTP_200_OK)
+        self.assertEqual(resp.headers["ETag"], '"v1"')
+        self.assertEqual(read_streaming_response(resp), b"thumb-v1")
+
     def test_metadata_version_param_uses_version(self) -> None:
         root = Document.objects.create(
             title="root",
diff --git a/src/documents/tests/test_version_conditionals.py b/src/documents/tests/test_version_conditionals.py
index fd24a2a51..81218ed4c 100644
--- a/src/documents/tests/test_version_conditionals.py
+++ b/src/documents/tests/test_version_conditionals.py
@@ -5,6 +5,7 @@ from django.test import TestCase
 
 from documents.conditionals import metadata_etag
 from documents.conditionals import preview_etag
+from documents.conditionals import thumbnail_etag
 from documents.conditionals import thumbnail_last_modified
 from documents.models import Document
 from documents.tests.utils import DirectoriesMixin
@@ -30,6 +31,7 @@ class TestConditionals(DirectoriesMixin, TestCase):
 
         self.assertEqual(metadata_etag(request, root.id), latest.checksum)
         self.assertEqual(preview_etag(request, root.id), latest.archive_checksum)
+        self.assertEqual(thumbnail_etag(request, root.id), latest.checksum)
 
     def test_resolve_effective_doc_returns_none_for_invalid_or_unrelated_version(
         self,
diff --git a/src/documents/views.py b/src/documents/views.py
index fde9a9106..1ff11f617 100644
--- a/src/documents/views.py
+++ b/src/documents/views.py
@@ -67,7 +67,6 @@ from django.views import View
 from django.views.decorators.cache import cache_control
 from django.views.decorators.csrf import ensure_csrf_cookie
 from django.views.decorators.http import condition
-from django.views.decorators.http import last_modified
 from django.views.generic import TemplateView
 from django_filters.rest_framework import DjangoFilterBackend
 from drf_spectacular.openapi import AutoSchema
@@ -124,6 +123,7 @@ from documents.conditionals import preview_etag
 from documents.conditionals import preview_last_modified
 from documents.conditionals import suggestions_etag
 from documents.conditionals import suggestions_last_modified
+from documents.conditionals import thumbnail_etag
 from documents.conditionals import thumbnail_last_modified
 from documents.data_models import ConsumableDocument
 from documents.data_models import DocumentMetadataOverrides
@@ -1564,7 +1564,12 @@ class DocumentViewSet(
 
     @action(methods=["get"], detail=True, filter_backends=[])
     @method_decorator(cache_control(no_cache=True))
-    @method_decorator(last_modified(thumbnail_last_modified))
+    @method_decorator(
+        condition(
+            etag_func=thumbnail_etag,
+            last_modified_func=thumbnail_last_modified,
+        ),
+    )
     def thumb(self, request, pk=None):
         resolved = self._resolve_request_and_root_doc(pk, request)
         if isinstance(resolved, HttpResponseForbidden):