The 28-year-old Birch joins fellow Montrealer Chris Boucher on the Raptors' roster after the Orlando Magic waived the undrafted Canadian earlier this week.
Birch tweeted the news on Saturday, in a thank-you message to Orlando.
"I'm lost for words on what the city of Orlando has meant to me & my family! Beyond grateful. I'm excited for the next chapter of my career with ?raptors," Birch wrote.
The six-foot-nine Birch is averaging 5.9 points and 5.1 rebounds in 19.8 minutes per game this season with Orlando, but had been hoping for a bigger role and more playing time with the Magic.
He's sure to get that with the Raptors, who've been weak in the frontcourt since Marc Gasol and Serge Ibaka left last off-season.
The Raptors announced the signing later Saturday ahead of the team's game in Cleveland. Birch could make his Raptors debut on Sunday in New York.
Raptors coach Nick Nurse coached Birch at the 2019 FIBA World Cup in China.
"I like him," Nurse said. "He did a good job for (Canada) and I think that's probably all I can say at this particular point. I really liked him in the World Cup. Hopefully he can grow and improve."
Birch was one of just two NBA players who suited up for Canada in China. Detroit Pistons guard Cory Joseph was the other.
Birch said then he enjoyed playing for Nurse, saying: "I'm doing things I have never done in the NBA. That's what a lot of guys don't understand. Coach Nurse gives you the confidence and freedom to do basically everything you want to in the system you have here. I'm lucky."
Birch has also said he's keen to be an Olympian and so planned to play for Canada in its last-chance qualifying tournament in Victoria this summer.
After his college career at Pittsburgh and UNLV, Birch played in the G League, Turkey and Greece before signing with the Magic in 2017.
Birch was part of a Magic team that lost in five games to the Raptors in the opening round of Toronto's run to the NBA title in 2019.
Boucher and Australian Aron Baynes are the other centres on Toronto's roster.