Hawks general manager Danny Ferry says the 6-foot-8 forward impressed the team by expanding his game.
Terms of the deal were not released, but the Atlanta Journal-Constitution reported that Scott agreed to a three-year deal worth $10 million.
"Mike has worked hard and continues to adapt, improve and expand his all-around game," Ferry said in a statement. "We are very happy to have him with us going forward and we look forward to his continued growth with our group."
Scott, a second-round pick from Virginia in 2012, averaged 9.6 points last season, up from 4.6 as a rookie. He started six of his 80 games and also averaged 3.6 rebounds.
Scott made 62 of 200 3-point attempts last season after missing his only 3 as a rookie. His five second-quarter 3s in Game 5 of the first round of the playoffs against Indiana were the second-highest total in any period in NBA playoff history.
The Associated Press contributed to this report.