A company official said terms, including length of deal and compensation, would not be disclosed.
The brand, which has had Harper as an endorser for five years, will beging selling Harper's first signature cleat, the Under Armour Harper One, in July. The company previously sold shirts with a phrase Harper filed to trademark -- "That's A Clown Question, Bro." Harper abandoned his pursuit of that trademark in 2014.
Under Armour signed Harper in April 2011, 10 months after the Nationals made him the first overall pick in the Major League Baseball draft. At the time the brand signed him, he was playing on the team's Single-A affiliate. He made it to the big leagues a year later.
At 19, he became the youngest player to be selected to the All-Star game and went on to win National League Rookie of the Year. In 2015, at the age of 23, became the youngest player to win an MVP award.
By retaining Harper, Under Armour continues its hold on recent MVP endorsers. It has Golden State Warriors guard and reigning NBA MVP Steph Curry signed through 2024 and PGA Tour Player of the Year Jordan Spieth under contract through 2025.
The brand also counts NFL MVP Cam Newton of the Carolina Panthers and NHL MVP Montreal goaltender Carey Price as endorsers, along with New England Patriots quarterback Tom Brady, Los Angeles Dodgers pitcher Clayton Kershaw, San Francisco Giants catcher Buster Posey and Olympic swimmer Michael Phelps.
Last month, Under Armour said it expected to do $5 billion in sales in 2016.