Yasuhiro Koseki (小関也朱篤, Koseki Yasuhiro; born March 14, 1992) is a Japanese competitive swimmer and breaststroke specialist. He won the 100-meter event at the 2013 Summer Universiade, 2013 East Asian Games, and the 2014 Pan Pacific Swimming Championships. He hold the Asian record in this event, set in February 2014.

Since 2010 Koseki studied at the Nippon Sport Science University, where he first specialized in the freestyle, and only later changed to breaststroke, motivated by Kosuke Kitajima.