ESPN's Adrian Wojnarowski reported the news.
Johnson spent his first two NBA seasons with the Indiana Pacers, who selected him in the second round of the 2018 draft.
The 24-year-old Missouri State product made a limited impact for the Pacers while also spending time with the Fort Wayne Mad Ants, the team's G League affiliate. He averaged 1.5 points and 2.2 rebounds per game while shooting just 35.6 percent from the field in 31 NBA appearances over the past two years.
His G League production was more promising. He averaged 20.1 points, 12.8 rebounds and 4.6 assists in 19 games for the Mad Ants during the 2019-20 campaign.
"[Johnson] works his butt off day-in and day-out. He's a great teammate and just awesome guy all around," Pacers forward Doug McDermott told reporters during last season. "Salute to Alize. He's a great player and he's got an unbelievable motor. We are really happy for him."
If he makes the Raptors' final roster, he'll provide forward depth behind starters OG Anunoby at the 3 and Pascal Siakam at the 4. It's unlikely he'll take on a major role barring a rash of injuries, though.
Johnson could also spend more time in the G League to continue his development. Raptors 905 are Toronto's affiliate.