"20th Century Boy" is a song by T. Rex, written by Marc Bolan, released as a stand-alone single on 2 March 1973.

[7] Although at first considered as its closing track, "20th Century Boy" was not featured on the album Tanx, released at the same time in early March.

[11] The song was met with a warm reception in Bolan's home country, where Chris Welch of Melody Maker noted that "guitars tremble, Marc howls, and it's 'gang awa' with another in his series of rhythmic entertainments".

[12] Charles Shaar Murray of NME thought it was the group's best since "Telegram Sam" with "Marc's thunder guitar majestically assaulting you."

However, Penny Valentine at Sounds, who had praised earlier T. Rex hits, now lamented that Bolan "hadn't extended his musical capabilities as far as he might have done.

The song was also used in the opening of the 2016 film The Purge: Election Year, and is the namesake of the manga series 20th Century Boys by Naoki Urasawa.