Olynyk was selected 13th overall by the Dallas Mavericks in the 2013 NBA draft. The same day, the Mavericks traded Olynyk to the Boston Celtics in exchange for Lucas Nogueira and two future second round picks. During his first summer league game, he scored 25 points with 9-of-13 shooting. He signed his rookie scale contract with the Celtics on July 7, 2013.