diff --git a/docs/source/command_line.rst b/docs/source/command_line.rst
index c085b63107b0..6dee0577cc42 100644
--- a/docs/source/command_line.rst
+++ b/docs/source/command_line.rst
@@ -96,6 +96,10 @@ Optional arguments
 
     Show program's version number and exit.
 
+.. option:: --O FORMAT, --output FORMAT {json}
+
+    Set a custom output format.
+
 .. _config-file-flag:
 
 Config file