Rachel Riley (born 11 January 1986) is an English television presenter. She co-presents the Channel 4 daytime puzzle show Countdown and its comedy spin-off 8 Out of 10 Cats Does Countdown. A mathematics graduate, her television debut came when she joined Countdown aged 22.
With an interest in popularising mathematics and the sciences, she has since co-presented The Gadget Show on Channel 5 (2013–14) and It's Not Rocket Science on ITV (2016). She was also a contestant on the BBC celebrity dance show Strictly Come Dancing in 2013.
In late 2018 and early 2019, Riley spoke about allegations of antisemitism in the Labour party, saying that leader Jeremy Corbyn had shared a platform with "Holocaust deniers and virulent anti-semites". Riley described Noam Chomsky as being "known to promote anti-semitism", based on his 1981 comments about Holocaust denier Robert Faurisson.