Barnes is the third player to win the annual award with Toronto, joining Damon Stoudamire (1995-96) and Vince Carter (1998-99).
Barnes received 378 points (48 first-place votes) from a global panel of 100 sportswriters and broadcasters.
Cleveland Cavaliers forward-center Evan Mobley finished in second place with 363 points. The 15 point difference was the closest since 2002.
Barnes averaged 15.3 points, 7.5 rebounds, 3.5 assists, 1.08 steals and 0.74 blocks in 35.4 minutes in 74 games, leading all rookies in minutes, ranking third in points and rebounds and finishing fifth in assists, steals and blocks.
A starter in every game he played, Barnes became the first rookie in Raptors history to record at least 1,000 points, 500 rebounds and 250 assists in a season.
He set the franchise rookie record for offensive rebounds with 195, which was a team high and ranked 15th among all NBA players.
Toronto selected Barnes with the fourth overall pick in the 2021 NBA Draft. Barnes played one season at Florida State, where he was named the 2020-21 ACC Freshman of the Year and Sixth Man of the Year.