The 26-year-old Hanson set a career high with 13 wins this year but his 4.48 ERA also was a career worst. The right-hander was 45-32 in four seasons with the Braves.
Walden had 32 saves in 2011, then lost his closer's role to Ernesto Frieri and was 3-2 with a 3.46 ERA and one save this year. He became superfluous when the Angels agreed this week to a $3.5 million, one-year contract with Ryan Madson.
Walden gives Atlanta another power arm for its bullpen. His fastball has been closed at close to 100 mph, and he had 48 strikeouts in 39 innings this year.
Atlanta also claimed right-hander David Carpenter off waivers from Boston.