The E331 series (E331系) was an electric multiple unit (EMU) train introduced experimentally from 2007 on commuter services on the Keiyo Line in Japan operated by East Japan Railway Company (JR East).

Only one trainset was built, and it differed significantly from other JR commuter EMUs in having articulated cars with shared bogies and using direct-drive motors.

[3] The lone set, AK1, consisted of two 7-car half-sets, and is formed as follows, with car 1 at the Tokyo end.

[1][3] Cars 2 to 13 had longitudinal seating, while the end cars 1 and 14 had transverse seating bays which could be rotated and rearranged longitudinally against the train sides to provide increased standing space during peak hours.

[1] Following extensive test running, the set entered revenue service on the Keiyo Line between Tokyo and Soga from the start of the revised timetable on 18 March 2007, used only on weekend services.