The move was retroactive to April 22.
Lidge has appeared in seven games this season, with two saves and an ERA of 5.14. He is in his first season with the Nationals after spending six years with Houston and four with Philadelphia.
He helped Philadelphia to a World Series title four years ago by converting all 48 of his save chances. The next year, however, he blew 11 saves and compiled an 0-8 record with a 7.21 ERA.
Lidge has appeared in 75 games during the last two seasons, during which he was hampered by elbow and shoulder injuries. He has struck out 796 in 601 innings during his career and has a lifetime ERA of 3.46.