diff --git a/tsdb/db.go b/tsdb/db.go
index 9c15dd4c25..3ddf73d21a 100644
--- a/tsdb/db.go
+++ b/tsdb/db.go
@@ -56,6 +56,8 @@ const (
 	// about removing those too on start to save space. Currently only blocks tmp dirs are removed.
 	tmpForDeletionBlockDirSuffix = ".tmp-for-deletion"
 	tmpForCreationBlockDirSuffix = ".tmp-for-creation"
+	// Pre-2.21 tmp dir suffix, used in clean-up functions.
+	tmpLegacy = ".tmp"
 )
 
 var (
@@ -1570,7 +1572,7 @@ func isTmpBlockDir(fi os.FileInfo) bool {
 
 	fn := fi.Name()
 	ext := filepath.Ext(fn)
-	if ext == tmpForDeletionBlockDirSuffix || ext == tmpForCreationBlockDirSuffix {
+	if ext == tmpForDeletionBlockDirSuffix || ext == tmpForCreationBlockDirSuffix || ext == tmpLegacy {
 		if _, err := ulid.ParseStrict(fn[:len(fn)-len(ext)]); err == nil {
 			return true
 		}
diff --git a/tsdb/db_test.go b/tsdb/db_test.go
index 6bedae9f35..e00f0a8f85 100644
--- a/tsdb/db_test.go
+++ b/tsdb/db_test.go
@@ -2795,15 +2795,20 @@ func TestOpen_VariousBlockStates(t *testing.T) {
 		require.NoError(t, os.Remove(filepath.Join(dir, metaFilename)))
 	}
 	{
-		// Tmp blocks during creation & deletion; those should be removed on start.
+		// Tmp blocks during creation; those should be removed on start.
 		dir := createBlock(t, tmpDir, genSeries(10, 2, 30, 40))
 		require.NoError(t, fileutil.Replace(dir, dir+tmpForCreationBlockDirSuffix))
 		expectedRemovedDirs[dir+tmpForCreationBlockDirSuffix] = struct{}{}
 
-		// Tmp blocks during creation & deletion; those should be removed on start.
+		// Tmp blocks during deletion; those should be removed on start.
 		dir = createBlock(t, tmpDir, genSeries(10, 2, 40, 50))
 		require.NoError(t, fileutil.Replace(dir, dir+tmpForDeletionBlockDirSuffix))
 		expectedRemovedDirs[dir+tmpForDeletionBlockDirSuffix] = struct{}{}
+
+		// Pre-2.21 tmp blocks; those should be removed on start.
+		dir = createBlock(t, tmpDir, genSeries(10, 2, 50, 60))
+		require.NoError(t, fileutil.Replace(dir, dir+tmpLegacy))
+		expectedRemovedDirs[dir+tmpLegacy] = struct{}{}
 	}
 	{
 		// One ok block; but two should be replaced.