The E231 series (E231系, E231-kei) is an electric multiple unit (EMU) train type used for commuter and outer-suburban services operated by East Japan Railway Company (JR East) in Japan since 2000.

Trains were manufactured by Tokyu Car Corporation, Kawasaki Heavy Industries and also at JR East's factory at Niitsu in Niigata Prefecture.

Other variants include the 11-car E231-500 series for the Yamanote Line, and the E231-800 series with 2,800 mm (9 ft 2 in) wide bodies and end doors for use on Tokyo Metro Tozai Line inter-running services.

These trains included transverse seating in some cars and were also equipped with toilets.

Production continued into 2006 with trains delivered for use on the Tōkaidō Main Line south from Tokyo.

From 2004 onward, two bilevel Green cars were inserted into all E231-1000 series ten-car sets.

The first full-production E231-0 series trains were introduced on the Chūō–Sōbu Line between February 2000 and November 2001, with 21 ten-car sets (101–121) based at Narashino Depot in Chiba[2] and 21 ten-car sets (20–25, 28–42) based at Mitaka Depot in Tokyo.

[4] New built E231-0 series sets were also delivered to Matsudo Depot between November 2001 and February 2005 for use on Jōban Line services between Ueno and Toride, and also Narita Line services to Narita, replacing the earlier 103 series fleet.

Each 11-car set originally contained two six-door SaHa E230 trailer cars (cars 7 and 10)[5] with six pairs of doors per side and bench seats that were folded up until 10 a.m. on weekdays to provide standing room only during the morning peak.

[16] To standardize door spacing ahead of the introduction of platform doors on all Yamanote Line stations by 2017, the two six-door cars in each set were subsequently replaced by a new build of four-door trailer cars between February 2010 and August 2011,[16] the first of which were delivered from JR East's Niitsu factory on 1 February 2010.

[4] From 7 September to 4 December 2009, E231-500 series set 502 carried an all-over brown livery to mark the 100th anniversary of the naming of Yamanote Line in collaboration with the Meiji Seika confectionery company.

From 16 January to 28 December 2013, Yamanote Line E231-500 series set 545 operated in an all-over uguisu light green livery to mark the 50th anniversary of the first appearance of 103 series EMUs in this colour scheme.

Ten- and five-car suburban sets based at Oyama and Kōzu depots for use on the following lines.

[4] As of 1 October 2018[update], 34 five-car sets (S-01–S-34) are based at Kōzu depot in Kanagawa Prefecture and 35 five-car sets (U2–U118) are based at Oyama depot in Tochigi Prefecture.

[36] Between June 2020 and September 2021, the trains were modified for driver-only operation (wanman (ワンマン, "one man")).

[4] On the evening of 5 August 2023, a Tokaido Line service led by E231-1000 series set S-14 struck a utility pole near Ofuna and lost power, reportedly due to getting caught by an overhead wire and pulling the pole.

[37] The accident resulted in four people, including the driver, sustaining minor injuries.