Roberts recently played for the Raptors' entry in the NBA Summer League in Las Vegas, where he averaged 10.3 points, 9.5 rebounds and 23.5 minutes in four games. He had 14 points and 12 rebounds in the opener July 10 against Sacramento.
Roberts split the 2014-15 season between the NBA Development League's Delaware 87ers and Santa Cruz Warriors, and the Philippines Basketball Association's San Miguel Beermen.
He averaged 10.7 points and 6.6 rebounds over four seasons at St. Joseph's and helped the Hawks claim the 2014 A-10 Tournament title.
Financial terms of the deal were not disclosed Tuesday.