The Thunder also received a 2029 second-round pick and another draft consideration from the Hawks in exchange for Vít Krejci.
Harkless' $4.5 million contract will slot into the $4.9 million disabled player exception granted to Oklahoma City last week after rookie Chet Holmgren's injury, according to ESPN's Bobby Marks.
The 6-foot-9 Harkless averaged 4.6 points and 2.4 rebounds in 47 games with Sacramento last season. He was traded by the Kings to the Hawks in July.
The 22-year-old Krejci was a second-round pick in the 2020 draft. The 6-foot-8 guard played in 30 games during his rookie season with Oklahoma City, averaging 6.2 points and 3.4 rebounds.
The Associated Press contributed to this report.