Zimmerman (ribcage) had been sidelined since July 7.
The Nationals also activated left-hander Sammy Solis (knee) from the disabled list, and optioned outfielder Michael Taylor and right-hander Lucas Giolito to Triple-A Syracuse.
Zimmerman, 31, is experiencing the worst season of his career. He entered Tuesday's contest with 12 homers and 38 RBIs while batting a career-low .221.
Solis has a 2.43 ERA in 25 games for Washington.