On Monday afternoon, Sullinger broke the news himself by tweeting "WE THE NORTH#0" and changed his Twitter avatar to a Raptors logo.
The Celtics rescinded a $4.4 million qualifying offer to Sullinger last week in order to clear the necessary cap space to sign Al Horford to a four-year, $113 million contract.
Sullinger could potentially start alongside center Jonas Valanciunas in Toronto's frontcourt next season.
The Raptors, who are coming off a franchise-best season in which they won 56 games and advanced to the Eastern Conference finals, had been seeking to upgrade at the power forward spot for a while.
Sullinger's scoring and playing time both dropped last season. He averaged 10.3 points, 8.3 rebounds and 2.3 assists over 23.6 minutes per game. Boston's crowded frontcourt and penchant for small ball lineups limited Sullinger's playing time, but the decreased minutes suggested the Celtics did not have confidence in leaning on Sullinger, who endured a rough postseason against an athletic Hawks front line.
Celtics coach Brad Stevens said Sunday that he believed Sullinger would find another home quickly.
"I think Jared's a really good player, and I think Jared will continue to be a really good player," Stevens said. "Obviously when we got Al [Horford], that made us make hard decisions. And the thing that stinks about that is having to move on and move forward, both for him and us. But he'll do great. Jared's one of the more talented guys being able to get the ball off the glass, and he's got a beautiful touch and he's a good guy, and so I think he'll do well.
"I'm hopeful this move will benefit him as well, and I think it will. We'll see where he lands, but I know there's a lot of interest in him based on how my phone's been ringing and everybody else's, so I'm sure he'll land where he wants to and do exceptionally well."
Sullinger earned $2.3 million last season before reaching restricted free agency.