"STAR21" was the name given to the Class 952/953 (952・953形) 9-car experimental Shinkansen train developed in 1992 by the East Japan Railway Company (JR East) in Japan to test technology to be incorporated in next-generation shinkansen trains operating at speeds of 350 km/h (217 mph) or higher. The name was an acronym for "Superior Train for the Advanced Railway toward the 21st Century".

The train consisted of nine cars arranged in two distinct "halves", with the Class 952 half-set consisting of four separate vehicles, and the Class 953 half-set consisting of five articulated vehicles using shared bogies.

Three different construction methods were used for the vehicle bodies. Cars 1 to 3 used welded hollow aluminium extrusions, cars 4 to 5 used brazed aluminium honeycomb panels, and cars 6 to 9 used an aircraft-style Duralumin fuselage construction.

The front-end design of the two driving vehicles (952-1 and 953–5) were slightly different, although both used a wedge-shaped profile with little lateral taper.

The external livery was light green for the Class 952 cars (including half of car 953–1), "snow" grey for cars 953–1 to 953–3, and beige for cars 953–3 to 953–5, with a light blue window band throughout the length of the train.

Internally, cars were fitted with passengers seats, arranged 2+2 abreast for standard class cars, and 2+1 abreast for the Green car (952-4). Five different types of lightweight seating design were tested.

The set, designated S5, was initially formed as follows with some cars unpowered.

Cars 1 to 3 were built by Nippon Sharyo, cars 4 to 5 were built by Hitachi, and cars 6 to 8 were built by Kawasaki Heavy Industries.