Celtics general manager Danny Ainge praised Walker's "hard work and skill development" over the summer in announcing the signing Friday.
Boston obtained the rights to the 6-foot-6 forward out of Kansas State for cash after he was chosen with the 47th pick by Washington in this year's NBA draft.
Walker had minor arthroscopic surgery on his right knee last month after injuring it in a pre-draft workout in June. He missed the Celtics' rookie camp. Walker had surgery on the same knee as a high school freshman.
In two seasons with the Wildcats, Walker averaged 15.3 points and 6.0 rebounds in 37 games.
Terms of the deal were not announced.
