Last season, the right-hander only threw 19 1-3 innings for the Philadelphia Phillies, going 0-2 with a 1.40 ERA and one save. He didn't pitch until late July because of elbow and shoulder problems in his right arm.
Lidge is a two-time All-Star who closed for the Phillies and Houston Astros during his 10-year major league career, going 26-31 with 233 saves and a 3.44 ERA in 592 appearances overall.
In Washington, Lidge joins a bullpen led by setup man Tyler Clippard, who was an All-Star in 2011, and closer Drew Storen.
The Nationals announced their deal with Lidge on Thursday.
