Harper, the No. 1 overall pick in the 2010 Draft, was hitting .250 with one homer and three RBIs in 20 games at Triple-A Syracuse.
"We thought that we needed to bring in an impactful left-handed bat," Nats general manager Mike Rizzo said. "He's swinging the bat extremely well right now and looks comfortable in left field."
Harper, 19, is expected to play Saturday when the Nationals will run another No. 1 overall pick on to the field in Stephen Strasburg. Chad Billingsley is expected to start for Los Angeles.
The move came after the Nationals placed third baseman Ryan Zimmerman on the 15-day Disabled List with a right shoulder injury.
"It was a team decision to help support the Major League club in a time we felt we needed the offensive support," Rizzo said on MLB Network.
Harper's numbers this season aren't overly impressive, but his bat has gotten hot in the last week or so -- in his last 10 games, Harper was hitting .290/.421/.419 with a double April 16 and a solo homer, his first at Triple-A, on April 22.
"He's had  at bats in Triple-A, but in his last six he's hitting .313, and in his last 10, he's hitting .290, so he's swinging the bat extremely well," Rizzo said on MLB Network. "His stroke is very good, he's running the bases well. He's an athletic, energetic player with big pop. We felt the last six games he's been hot and centering the ball.
"We feel this guy will help us and play a good sound left field and give us a little thunder in the lineup," he added.
Much of Harper's struggles this season have come against left-handed pitching, where he's batting .190 in 21 at-bats. Overall, he's 9-for-31 with seven walks in his last 10 games dating back to April 16.
Rizzo said calling up Harper due to injuries "wasn't the coming-out party for Bryce we had in mind. This isn't the optimal situation developmentally."
Harper, ranked by MLB.com as the No. 2 prospect in baseball behind the Rays' Matt Moore, signed a $9.9 million contract in August 2010 and has been one of the biggest stories in the Minors ever since. His path through the Minors and his eventual debut has been debated by fans and the media constantly, although many were surprised the Nationals pulled the trigger before May.
"Have to say it came earlier than I thought," MLB.com prospects expert Jonathan Mayo Tweeted following the news, "but here comes Bryce."
Rizzo admitted the callup on April 27 wasn't in the original plans.
"We had the developmental plan in place for Bryce, and we still believe in the plan, but we had to deviate from the plan a little but because of the circumstances," he said.
Rizzo said the Nationals are confident the 19-year-old will mesh well with a team that currently is 14-5 and owns the best record in the National League.
"This is a kid who has always played above his peer group, always played with older players, and we feel he has the confidence level and self-assuredness," Rizzo said. "He has a special skill set. He may not be perfectly ready for the big leagues -- I would have loved for him to get 350 at-bats in the Minors."
The move may not be temporary, either -- Rizzo said Harper could remain in the Majors when Zimmerman returns from the DL, depending on how well he plays.
"Zimmerman will come off the DL early in May and we'll make some determinations at that time," Rizzo said. "If he's playing well, then he'll stay on the ballclub. But if we feel we need to re-address the plan, we'll send him back down."
Rizzo reiterated that the club believes sending Harper back to Syracuse wouldn't hinder the outfielder's confidence or development, a potential scenario that many fans hinted could happen if the College of Southern Nevada product was rushed to the Majors too quickly.
"We fell it wouldn't hinder his progress or retard his development if that happened," Rizzo said, citing Tigers great Al Kaline and Angels prospect Mike Trout as other players who were sent down after early Major League stints.
The Nats are on the road in Los Angeles on Saturday and have a 9:10 p.m. ET game scheduled with the Dodgers at Dodger Stadium. Strasburg, who carried the hype of Harper in his brief Minor League career, will start, and Rizzo said the team is determined on keeping a winning squad on the field this season after years of disappointment in D.C.
"We're here to win games, and Bryce will help us win games," he said. "We're really excited about where we're at and we think Bryce Harper will help us in the Majors Leagues and be an impact player in the near future."