The 28-year-old guard averaged a career-best 15.5 points per game for the Raptors, appearing in 80 of 82 games and helping Toronto to a franchise-record 49 wins.
He got 78 first-place votes from a panel of 130 writers and broadcasters who cover the NBA regularly and becomes the first Raptor to win ever win the award.
Only two other Toronto players — rookies of the year Damon Stoudamire and Vince Carter — have ever won end of the season NBA awards.
Isaiah Thomas of the Boston Celtics finished second to Williams and Jamal Crawford of the Los Angeles Clippers was third in voting.