The 32-year-old McLouth has right shoulder inflammation. He's batting .179 in 79 games, including 33 starts, with one homer and seven RBIs. The move, announced Monday, is retroactive to Saturday.
Washington filled the roster spot by recalling outfielder Steven Souza Jr. from Triple-A Syracuse.
This will be Souza's second stint this season with the Nationals. He previously went 1-for-8 in eight games.
Souza leads the International League with a .354 batting average.