The struggling sportswear giant Under Armour said Tuesday that its chief executive, Kevin Plank, was stepping down from the role he had held since starting the company in his grandmother’s basement more than 20 years ago.

Under Armour, which largely focuses on making high-tech shoes and apparel for athletes, has faltered while the revenues of its rivals Nike and Adidas have grown thanks to surging consumer interest in athleisure wear.

The company, which had $5 billion in sales in 2018, has seen its once-robust profit turn into net losses of more than $46 million in each of the last two fiscal years. Last year, it cut around 400 jobs to streamline a business suffering from slowing growth.

Under Armour’s stock peaked around $52 in September 2015, but is currently trading around $20. It rose 2.6 percent in early trading Tuesday after the announcement about Mr. Plank, who will remain at the company in an executive role.