The three-year, $12 million contract was agreed to last week but could not be announced until Thursday.
Sefolosha, a native of Switzerland, reached agreement after the Hawks cleared more than $15 million in salary cap space by trading guard Lou Williams and the draft rights to Brazilian center Lucas Nogueira for Toronto forward John Salmons. The Hawks could have about $11.5 million for additional signings if they pay the $1 million buyout of Salmons' $7 million salary.
The 6-foot-7 Sefolosha averaged 6.3 points as a starter in 61 games last season for Oklahoma City. The guard-forward is known as a strong defender but his 3-point percentage fell from 41.9 percent in 2012-13 to 31.6 percent.