Monetary terms of the deal were no disclosed.
Prior to being released, Niemi was under salary arbitration. He was awarded a $2.75 million judgment on Saturday. The Hawks had 48 hours to consider the ruling, but opted to release Niemi, who is now an unrestricted free agent.
Turco previously played for the Dallas Stars, where he went 22-20 with a 2.7 goals-against average and .913 save percentage and four shutouts.
By comparison, Niemi was 26-7 with a 2.2 GAA and .912 save percentage. Niemi had seven shutouts.