The team made the move before Monday night's game against Colorado, which happens to be Bryce Harper bobblehead night at Nationals Park.
Ryan Zimmerman, who had been playing in Harper's spot in left field to help with his chronic shoulder problems, will return to third base. To make room on the roster, the Nationals optioned left-handed reliever Xavier Cedeno to Triple-A Syracuse.
Before making his outfield debut on June 3, Zimmerman had played 1,100 games as a third baseman. But his move to the outfield was necessitated by his throwing problems this season and the need for a strong hitter to play left field while Harper recovered from thumb surgery.
The 21-year-old Harper, the 2012 National League Rookie of the Year, was hitting .289 with one home run and nine RBIs in 22 games before getting hurt. He played five rehabilitation games in the minors and hit .643, including three homers in a Double-A game over the weekend.
Cedeno pitched in one game during his third stint in the majors this season. He threw a scoreless inning Saturday against the Chicago Cubs.
The Associated Press contributed to this report.