This designation was allocated to the tandem, dual-control, trainer version of the Yak-15. The pupil's cockpit was added in front of the original cockpit, and a new streamlined canopy was designed to cover both, with a separate sideways-hinged hood for each cockpit. To make room for the front cockpit the guns were removed, and the Yak-21 was unarmed. Fuel capacity was unaltered. The single prototype had callsign 101, believed in Yellow, and a capital V (looking like B in Russian) was painted on the rudder. Flight testing began in May 1947, but by this time it had been decided to build the Yak-17UTI with a nosewheel undercarriage.