Rachel Annabelle Riley (born 11 January 1986) is an English television presenter and mathematician. 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.