diff --git a/Documentation/git-repack.txt b/Documentation/git-repack.txt
index b9c02ce481..cde7b4441d 100644
--- a/Documentation/git-repack.txt
+++ b/Documentation/git-repack.txt
@@ -128,6 +128,12 @@ other objects in that pack they already have locally.
 	with `-b` or `repack.writeBitmaps`, as it ensures that the
 	bitmapped packfile has the necessary objects.
 
+--unpack-unreachable=<when>::
+	When loosening unreachable objects, do not bother loosening any
+	objects older than `<when>`. This can be used to optimize out
+	the write of any objects that would be immediately pruned by
+	a follow-up `git prune`.
+
 Configuration
 -------------