Joshua, who is set for his first world title fight against American Charles Martin in April, joins a roster of athletes that also includes fellow countryman Andy Murray and American football icon Tom Brady.
But it is the fortunes of golfer Jordan Spieth, basketball star Steph Curry and middleweight boxer Saul 'Canelo' Alvarez, all of whom have risen to the top of their sports since signing with the brand, that Joshua will most want to emulate.
"Having Under Armour in my corner is a huge show of support from one of the world's leading sports brands," he said.
"I couldn't ask for a better partner to help my performances, whether I'm training or in the ring; I'll relish every challenge we'll face in the future."
Joshua, a gold medal winner at London 2012, has quickly gained notoriety as one of the most exciting boxers in the heavyweight division with 15 wins from 15 fights, all by way of knockout.
He will now be kitted out with in-ring and training apparel from the US brand who have cited an impressive roster of sponsored athletes as one of the driving forces behind rising revenues that have seen it claim the status as the second biggest sportswear company in America ahead of Adidas and behind Nike.
Under Armour's vice president and managing director for Europe, Chris Bate, said: "Anthony Joshua encapsulates what our brand is about. He's a man that has achieved greatness, and will continue to do so, through relentless passion, drive and determination.
"From the first sparring session to the last bell, Anthony will have our innovative footwear, apparel and training tools at his disposal. We're proud to help Anthony every step of the way, as he establishes himself as one of the world's greatest boxers."