The news broke just as Barnes was cleared to play in Game 4 of the Raptors' playoff series against the Philadelphia 76ers. He had missed two games of the series with a sprained ankle as Toronto fell behind 3-0 in the best-of-seven series.
A video tribute showed former Raptors great Vince Carter making the announcement to Barnes and his Toronto teammates after their morning shootaround.
"We are incredibly proud of Scottie, and are thrilled and grateful that his hard work has been recognized with this honour," said Raptors vice-chairman and president Masai Ujiri in a statement. "What you see on the court is exactly who Scottie is: enthusiastic. Joyful. Athletic. Skilled, and a winner.
"We — and our fans — loved seeing his development through this season, and we can't wait to see what the future brings."
Barnes, the No. 4 pick in the 2021 draft, becomes the third Raptor to win rookie honours after Damon Stoudamire (1996) and Carter (1999).
Stoudamire was also featured in the video tribute at Scotiabank Arena. Barnes stood at centre court during the pre-game ceremony and Ujiri presented him with the rookie of the year trophy to a loud ovation from Toronto fans.
Barnes stepped forward and held the award over his head to an explosion of cheers from the arena, hyping the crowd up ahead of the critical game.
Raptors head coach Nick Nurse wasn't even sure Barnes would play against the Sixers on Saturday.
"We're still just waiting. He's upstairs right now getting his fitness test to see how he's doing," said Nurse in a pre-game news conference. "He did participate in the parts of practice. We shall see shortly."
Nurse didn't have to wait long to find out if Barnes was healthy enough to play.
The crowd rose to its feet again with 6:11 left in the first quarter as it was announced that Barnes was checking into the game, replacing OG Anunoby. He made an immediate impact, grabbing a defensive rebound on the first possession he was on the court.
The 20-year-old Barnes averaged 15.3 points and 7.5 rebounds on 49.2 per cent shooting in his first season with the Raptors. He averaged 35.4 minutes per game, first among all rookies, and was the only rookie to rank in the top five in points, rebounds, assists, steals, and blocks.