Helicopter Sea Combat Squadron 21 (HSC-21), nicknamed "Blackjacks", is an aviation unit of the United States Navy based at Naval Air Station North Island.

HSC-21 flies the Sikorsky MH-60S Seahawk helicopter.

HSC-21 was established as Helicopter Combat Support Squadron 11 (HC-11) "Gunbearers" on 1 October 1977 at Naval Air Station North Island, San Diego, California (United States), to provide logistics support for the units of the United States Pacific Fleet.

During Operation Iraqi Freedom HSC-21 had shore base detachment as a part of the 2515th Naval Air Ambulance Detachment, Udari Army Airfield, Kuwait.

This United States Navy article is a stub.