Aboard a starship or on a facility, an assistant chief of security is a senior security officer who is assigned to assist the chief of security in running the facilities security division. If the chief of security is on leave or incapacitated, then the assistant chief is trained to take over.

In 2266, Ensign Helga Litwak served as assistant chief aboard the USS Constellation. (TOS - The Brave and the Bold, Book One novella: The First Artifact)

In the late 2260s, Lieutenant Beranardi al Auriga served as assistant chief of security aboard the USS Enterprise, under Lieutenant Commander Mandala Flynn. (TOS novel: The Entropy Effect)