The Cavaliers traded the point guard and forward Channing Frye to the Lakers on Thursday for guard Jordan Clarkson and forward Larry Nance, according to the source.
The Lakers also get the Cavaliers' protected first-round pick in 2018. The trade deadline is at 3 p.m. ET Thursday.
Thomas was acquired from the Celtics in August in the bigger deal for Kyrie Irving, who had said he wanted out of Cleveland.
The Cavs traded six players, including Thomas, Dwyane Wade and Derrick Rose, and two future draft picks in moves designed to not only help them in the short term but could potentially help keep LeBron James beyond this season.
ESPN and the Associated Press contributed to this report.