Weather forecasts called for thunderstorms throughout the afternoon, forcing the postponement.
According to manager Jim Riggleman, the Nationals will use the day off to skip scheduled starter Jordan Zimmermann in the rotation in an attempt to limit his innings. Zimmermann, who underwent Tommy John surgery in August 2009, is not hurt.
The Nationals and Pirates played only Monday's opener of their two-game series, which Washington won, 4-2. The Pirates return to Washington to play a series from July 1-3.
The Nationals now head to New York, where they'll open a two-game set with the Mets on Wednesday at Citi Field.
The rotation will continue as originally planned, with Tom Gorzelanny starting the series opener and Livan Hernandez getting the nod on Thursday.
The Nationals open Interleague Play on Friday in Baltimore, with Jason Marquis getting the start in the opener, John Lannan starting on Saturday, and Zimmermann keeping his spot on Sunday.
Zimmermann has already thrown 48 innings this season, more than half of his career high with Washington, which came in 2009 when he made 16 starts for the Nats and also spent time in the Minors.
Steven Miller is an associate reporter at MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.