Thomas Leezer (born 26 December 1985) is a Dutch retired road bicycle racer, who competed between 2004 and 2020, for the Van Vliet–EBH Advocaten, Rabobank Continental Team and Team Jumbo–Visma squads.

Leezer grew up in 's-Gravenzande, where his father was a veterinary surgeon.

During his professional career, Leezer took one individual victory on stage six of the 2013 Tour de Langkawi, winning after a 7-kilometre (4.3-mile) solo move. He also competed at eight Grand Tours – including wearing the mountains classification jersey at the 2009 Vuelta a España – as well as the team time trial at the UCI Road World Championships on four occasions, and the road race at the 2016 UCI Road World Championships. At the latter race, he almost became World champion after escaping from the leading group and was only overtaken a few hundred meters ahead of the finish line.

Following his retirement from cycling, Leezer co-founded Dutch cycling clothing brand Velor, and in 2025, became the national junior coach for the Royal Dutch Cycling Union.