Geraint Howell Thomas, ( GHERR-eyent, Welsh: [ˈɡɛraint]; born 25 May 1986) is a Welsh professional racing cyclist who rides for UCI WorldTeam Ineos Grenadiers, Wales and Great Britain. He is one of the few riders in the modern era to achieve significant elite success as both a track and road rider, with notable victories in the velodrome, in one-day racing and in stage racing. On the track, he has won three World Championships (2007, 2008, and 2012), and two Olympic gold medals (2008 and 2012), while on the road he won the 2018 Tour de France becoming the first Welshman and third British rider to win it.

His early successes were in track cycling, in which he was a specialist in the team pursuit. He won three World Championships and was Olympic gold medallist twice, in 2008 and 2012. Thomas had an early win on the road at the 2004 Paris–Roubaix Juniors and later had a senior victory at the 2010 British National Road Race Championships. Leaving track cycling to focus solely on the road, he subsequently found success in both one-day/classic races such as the 2014 Commonwealth Games road race and the 2015 E3 Harelbeke, and in one week stage races, most notably at the 2016 Paris–Nice, the 2017 Tour of the Alps, the 2018 Critérium du Dauphiné, the 2021 Tour de Romandie and the 2022 Tour de Suisse.

In cycling's grand tours, Thomas was initially a lead domestique to Chris Froome in his victories. He won the first stage of the 2017 Tour de France, an individual time trial, to become the first Welshman to wear the Tour's yellow jersey. He later crashed in that race, as well as in the 2017 Giro d'Italia. Thomas became the first Welshman to win the Tour de France when he won the race in 2018. He gained the yellow jersey by winning stage 11, extended his lead by winning stage 12, and retained the lead for the remainder of the race. In the same year he won the BBC Sports Personality of the Year Award, becoming the first Welshman to win the award since Ryan Giggs in 2009. In 2019, he confirmed his Grand Tour pedigree when he reached the podium again, having finished runner-up in the Tour de France behind Team Ineos teammate Egan Bernal. In 2022, Thomas became the first Welshman to win the Tour de Suisse and later that year recorded another podium finish at the Tour de France, placing third. A fourth grand tour podium, and a first outside of the Tour de France, was won in 2023 as Thomas took second place in the Giro d'Italia, losing the race lead to Primož Roglič on the penultimate day. A fifth grand tour podium was secured in the following Giro, less than half a minute behind runner up Danny Martinez, but almost ten minutes behind winner Tadej Pogacar. After the race, Thomas suggested he would no longer race for general classification at the grand tours

Notable for his all-round ability and adaptability rather than mastery of one specialism, Thomas has been competitive in individual time-trials, on the cobbles, in the Spring classics and in the mountains of Grand Tours, where he was the first rider in the history of the Tour de France to win at Alpe d'Huez while in yellow.