Under Armour, Inc. is an American sportswear company that manufactures footwear and apparel headquartered in Baltimore, Maryland, United States. Under Armour was founded on September 25, 1996, by Kevin Plank, a then-24 year old former special teams captain of the University of Maryland football team. Plank initially began the business from his grandmother's basement in Washington, D.C. He spent his time traveling along the East Coast with nothing but apparel in the trunk of his car. His first team sale came at the end of 1996 with a $17,000 sale. From his grandmother's Washington, D.C.-area rowhouse, Plank moved to Baltimore.

After graduating from the University of Maryland, Plank developed his first prototype of the shirt, which he gave to his Maryland teammates and friends who had gone on to play in the National Football League, and soon improved the design, creating a new T-shirt built from microfibers that wicked moisture away. Major competing brands including Nike, Adidas and Reebok would soon follow with their own moisture-wicking apparel. Plank opted to use the British spelling "armour" in the company name because the toll-free vanity number was still available for that version. That year, Under Armour launched several new apparel lines including ColdGear, TurfGear, AllseasonGear, and StreetGear. By the end of 1996, Under Armour had sold 500 Under Armour HeatGear shirts, generating $17,000 for the company. In 1997, Plank had $100,000 in orders to fill and found a factory to make the shirts.