diff --git a/source/reference/command/shardCollection.txt b/source/reference/command/shardCollection.txt
index a55a0fee8c7..121388bd9d3 100644
--- a/source/reference/command/shardCollection.txt
+++ b/source/reference/command/shardCollection.txt
@@ -38,6 +38,12 @@ shardCollection
          use the specified field as a :ref:`hashed shard key
          <sharding-hashed-shard-key-internals>` .
 
+   :param boolean unique:
+
+      When true, the ``unique`` option ensures that the underlying
+      index enforces a unique constraint. :term:`Hashed shard keys
+      <hashed shard key>` do not support unique constraints.
+
    :param integer numInitialChunks:
 
       .. versionadded:: 2.4