Speedo sponsored Michael Phelps for most of his career. The company signed him to his first major endorsement deal when he was 15, around the time he became the youngest American male swimmer to make an Olympic team. Speedo remained closely associated with Phelps through his rise from teenage talent to the most decorated Olympian in history.
The partnership became especially visible during the 2004 Athens and 2008 Beijing Olympics. After Phelps won eight gold medals in Beijing, Speedo awarded him a $1 million bonus, which he pledged toward establishing the Michael Phelps Foundation. His Speedo LZR Racer swimsuit also became part of the conversation surrounding the many world records set at the 2008 Games.
Phelps and Speedo ended their long relationship after the 2012 Olympics. He later signed with Under Armour, so Speedo was not his sponsor for every final season. Arena and TYR are major swimwear brands, while Nike has sponsored many elite athletes, but Speedo is the canonical answer for Phelps's longest-running and most famous career sponsorship.