Encarnacion belted a club record 16 home runs last month and batted .281 with five doubles, one triple and 33 RBIs. He scored 26 runs and walked 16 times over 30 games to claim the honour for the first time.
Encarnacion had five multi-homer games in May. He was just shy of Carlos Delgado's club record of 34 RBIs in a month set in June 2003.
Others receiving votes included Miguel Cabrera (.380, 8 HR, 34 RBI), Detroit teammate Victor Martinez (.372, 9 HR, 21 RBI) and Baltimore's Nelson Cruz (.339, 13 HR, 27 RBI).
Jose Bautista was the last Toronto player to win the award. He took the honour in June 2012.