Harper was chasing a deep fly ball in the fifth inning when he crashed into a hard portion of the wall that features a scoreboard and does not have padding.
He remained on the ground for several minutes before walking off as the Nationals went on to beat the Dodgers 6-2.
The 20-year-old Harper is enjoying a strong start to the season after making waves throughout baseball in 2012 when he was named an All-Star and won the National League Rookie of the Year.
The first overall pick in the 2010 Major League Draft, Harper is delivering on expectations and is batting .303 with 10 home runs and 21 RBIs.
He was playing in his second game back after missing two games with a toe injury.