Williams, who is the first Raptors player to win the award, had 78 first-place votes and 502 total points from a panel of 130 sportswriters and broadcasters in the United States and Canada.
Boston Celtics guard Isaiah Thomas finished second in the voting (33 first-place votes, 324 points), and two-time winner, Jamal Crawford of the Los Angeles Clippers, was third (eight first-place votes, 131 points).
In unofficial balloting, four of the six SI.com NBA experts chose Williams to win the award.
SI’s Ben Golliver said of Williams that the Raptors "put questions about his surgically-repaired knee behind him this season, averaging 15.5 PPG and 2.1 APG while serving as a stabilizing force when starting guards DeMar DeRozan and Kyle Lowry missed time due to injury."
Williams averaged a career-high 15.1 points in 80 games this season, all off the bench.
Williams, 28, shot 40 percent from the field and 86 percent from the free-throw line in his first season with the Raptors, who won the Atlantic Division for the second straight year.
Toronto trail one game to none in their Eastern Conference first-round playoff series against the Washington Wizards.
- Scooby Axson