The 34-year-old model will lead a new campaign targeting health-conscious women. Husband and NFL star Tom Brady has represented the brand since 2010, and holds a minority ownership stake in the company.
Under Armour released a teaser video of Bündchen on Tuesday that sees her model some of the apparel. Further details will be revealed on September 4.
Bündchen will represent the brand alongside surfer Brianna Cope, tennis player Sloane Stephens, ballet dancer Misty Copeland and skier Lindsey Vonn. The model holds a number of other endorsements with brands such as H&M, Chanel and Balenciaga, and reportedly earns $128,000 a day.