He'll be joined in the starting lineup by Nationals' teammates Daniel Murphy and Ryan Zimmerman.
Harper will be in Miami for the All-Star weekend after being named a starting outfielder for the National League.
The Los Angeles Dodgers, who have the best record in the National League, will not have any All-Star starters.
A pair of Astros are part of the AL's starting infield after shortstop Carlos Correa and second baseman Jose Altuve were chosen in the balloting, along with Cleveland third baseman Jose Ramirez and Oakland first baseman Yonder Alonso.
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Tampa Bay's Corey Dickerson was selected as the designated hitter.
With the addition of the reserves and pitchers, the Indians, Yankees and Astros each had five players named to the AL All-Star squad.
This will be Zimmerman's second career all-star game.
In the National League, only the Nats had five players make the roster, as right-handers Max Scherzer and Stephen Strasburg were selected.
His only other appearance came in 2005 when he was a National League reserve third baseman. Rockies third baseman Nolan Arenadodo and Reds shortstop Zack Cozart fill out the infield, with Giants catcher Buster Posey behind the plate. He leads all National League first baseman in batting average and hits.
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