Kelly, 26, appeared in 16 games for the Hawks last season, averaging 1.6 points and 1.1 rebounds in limited play.
The 6-foot-11, 230-pound Kelly was originally signed by the Hawks in September 2016. He was waived and re-signed a few more times last season before re-signing a multi-year contract on Feb. 24.
Kelly was drafted by the Los Angeles Lakers in the second round of the 2013 NBA Draft. He owns career averages of 6.0 points and 3.1 rebounds in 163 games (59 starts).