The Seibu 20000 series (西武20000系) is an electric multiple unit (EMU) train type operated by the private railway operator Seibu Railway on commuter services in the Tokyo area of Japan.

As of 1 April 2017[update], the fleet consists of eight ten-car sets and eight eight-car (20050 series) sets, based at Kotesashi, Minami-Iriso, Musashigaoka, and Tamagawa-Josui depots for use on Seibu Shinjuku Line and Seibu Ikebukuro Line services.

[3] From 8 October 2016, eight-car set 20158 operated in a special Galaxy Express 999 vinyl wrapping livery.

[4][needs update] From 15 January 2018, two ten-car sets, 20104 and 20105, are scheduled to operate in a special "L-train" livery consisting of the Saitama Seibu Lions baseball team colour of dark blue with Seibu Lions logos.

The sets re-entered service on the Shinjuku, Haijima, Ikebukuro, and Sayama lines from then.