Phelps first signed on with Speedo at the age of 15
Michael Phelps is returning to the pool today
, but no one knows what he is going to wear. The Olympic swimmer's endorsement deal with Speedo ended at the end of last year, and they have parted ways. He had worn Speedo in competition since signing with the company in '01. Speedo confirmed that the partnership ended in a statement, saying, "When Speedo signed Michael to his first endorsement deal at age 15, becoming his first-ever sponsor, we believed that he had the potential to change the sport of swimming forever. He has certainly achieved that, becoming undeniably the most successful Olympian of all time. ... Though the partnership with Michael has ended, he will always be a part of Speedo's history and we wish him well in the future." Phelps, who retired following the '12 London Games, is returning to competition this weekend at the Arena Grand Prix in Mesa, Ariz. It is unclear what brand's swimsuit he will wear. Phelps is not in talks with Speedo about a new deal, and efforts to reach Phelps' reps at Octagon were unsuccessful. Phelps has an apparel deal with Under Armour, which is headquartered in his hometown of Baltimore, but the company has not launched a pro swimwear line. It partnered with California-based swimwear company Finis last year in a deal to have Finis' sponsored clubs, high school and college teams wear Under Armour warm ups and apparel. Another possibility for Phelps would be to sign with Arena, which has ratcheted up its investment in swimming since signing an eight-year sponsorship of USA Swimming in '12. There also is Tyr, which has had endorsements with several Olympians. The Phelps-Speedo relationship is considered one of the most significant in the history of swimming. Phelps' agent, Peter Carlisle, negotiated a deal with Speedo in '03 that would see the company pay Phelps a $1M bonus if he tied Mark Spitz' record of seven Gold Medals at a single Olympics. Phelps collected the bonus after winning eight Gold Medals at the '08 Beijing Games and used it to launch his charity, the Michael Phelps Foundation. He, Natalie Coughlin and Ryan Lochte were the face of Speedo's marketing in the sport for most of the past decade.