Bett began to compete on the international track and field circuit in 2015 and had wins at the Savo Games and Lappeenranta Games in Finland. A personal best of 48.29 seconds at the 2015 Athletics Kenya World Championship Trials brought him selection for the national team and raised him to third on the global rankings. At the 2015 World Athletics Championship in Beijing, Bett achieved his biggest result, winning the 400 metres hurdles event with a time of 47.79 seconds. Bett's win was Kenya's first in the history of the World Championships, for an event shorter than 800 metres. Bett's Beijing run was the fastest time for the distance in the whole of the 2015 season.