Nationals superstar and reigning NL MVP Bryce Harper has signed a 10-year endorsement deal with Under Armour that's believed to be the biggest such contract ever signed by an MLB player, ESPN's Darren Rovell reports. Harper was already with Under Armour -- since not long after he was drafted -- so this amounts to a contract extension.

Harper himself took to Instagram to acknowledge the news ...

Under Armour will not be disclosing the precise terms of the deal.

Harper's still just 23 years of age, and he's off to a tremendous start in 2016. Given his youth, on-field excellence, and telegenic appearance, Under Armour will likely be very pleased with their decision. More generally, greater commercial visibility for the game's incredible crop of young stars is very much a good thing for baseball.

Everybody wants a piece of Bryce Harper.
Everybody wants a piece of Bryce Harper. (USATSI)