The thirteenth series of the British science fiction television programme Doctor Who, known collectively as Flux, was broadcast from 31 October to 5 December 2021. The series is the third and last to be led by Chris Chibnall as head writer and executive producer. It is the thirteenth to air following the programme's revival in 2005, and the thirty-ninth season overall. The series, initially announced in November 2019, was the last to be broadcast on Sunday nights, continuing the trend set by the previous two series. It was followed by three associated specials, all of which aired in 2022.

Christopher Antony Chibnall is an English television writer and producer, best known as the creator and writer of the award-winning ITV mystery-crime drama Broadchurch (2013-17) and as the third showrunner of the 2005 revival of the BBC sci-fi series Doctor Who (2018–22). Chibnall wrote five episodes of the series under previous showrunners Russell T Davies and Steven Moffat, and he was also the head writer for the first two series of the spinoff Torchwood (2006-08).