Karl Ravech and Tim Kurkjian explain how Bryce Harper's 10-year endorsement deal with Under Armour speaks to the popularity of baseball. (1:38)

Washington Nationals star Bryce Harper has signed a 10-year extension with Under Armour that is the largest endorsement deal in history for a baseball player, a source with knowledge of the deal told ESPN.

Under Armour announced Tuesday that Harper signed a multiyear extension but said that terms, including the length of the deal and compensation, would not be disclosed.

The exact amount of Harper's deal wasn't immediately known, but a source said it surpasses the largest overall endorsement deal in the sport when Nike agreed, in 2005, to pay Alex Rodriguez a little more than $1 million a year for 10 years. The deal also includes stock, the source said.

Bryce Harper's new deal with Under Armour runs through 2026, making him the latest superstar signed long term with the company. Under Armour has Stephen Curry signed through 2024 and Jordan Spieth signed through 2025. AP Photo/Luis M. Alvarez

The brand, which has had Harper as an endorser for five years, will begin 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 Class A affiliate. He made it to the big leagues a year later.

At age 19, Harper 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 age 23, he became the youngest player to unanimously 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 Stephen Curry signed through 2024 and PGA Tour Player of the Year Jordan Spieth under contract through 2025. Harper's new deal runs through 2026.

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 make $5 billion in revenue in 2016.