331st may refer to:
331st Air Expeditionary Group, inactive United States Air Force unit 331st Fighter-Interceptor Squadron, former unit of the United States Air Force 331st Guards Airborne Regiment, formation of the Russian Airborne Troops, part of the 98th Guards Airborne Division 330th Rifle Division (Soviet Union), former infantry division of the Red Army in summer 1941
See also 331 (number) 331, the year 331 (CCCXXXI) of the Julian calendar