The line of "lifestyle apparel" will bear the image and famous mottos of Ali including "King Of Boxing," "Cassius Clay," "Float Like A Butterfly Sting Like A Bee," "The Greatest Of All Time," "Rumble In The Jungle" and "Thrilla In Manila."
Vice president for Under Armour, Glenn Silbert, said that it made sense to use such an iconic sports person with their brand.
"He's an iconic hero from the past and a true innovator," said Silbert.
"We wondered what Ali would have been like if he had a brand like ours when he was getting started and how great it could have been. That has been the mentality of our design team throughout this process."
Ali, 73, retired in 1981. Since then, he has opened up about his battle with Parkinson's Disease.
Due to his limited capacity, he will not be able to promote the deal himself.