The five-year agreement represents the American company's largest European team sponsorship and Press Association Sport understands it could be worth up to ?10m-a-year to the club.
Beginning with the 2012/13 season Under Armour, who currently supply the Wales national rugby team, will replace Puma in providing Tottenham's playing kit and training wear, as well as replica shirts.
Speaking on the deal, Tottenham chairman Daniel Levy said: "We are delighted Under Armour will become our new technical partner from 2012 onwards.
"They are an extremely ambitious brand with global aspirations, making them ideal partners for Tottenham Hotspur."
Source: PA
Source: PA