Johnson, 25, is 6-foot-6 and appeared in 61 games with the Raptors last season, averaging 4.4 points and 2.5 rebounds. He has been playing in the G League with the South Bay Lakers after signing with the club Nov. 15.
He was originally drafted by the Pistons with the No. 8 overall pick in 2015 and has made a name for himself with strong perimeter defense.
Johnson actually signed a non-guaranteed deal with the Bulls for training camp, but was waived before the season. He appeared in all four preseason games with the Bulls before that, averaging 3.3 points and 2.5 rebounds.
He has spent the season on the list of top available veterans on Hoops Wire.
Chicago visits Cleveland on Tuesday night. Tipoff is scheduled for 7 p.m. EST.