Phelps went 8-for-8 in Beijing and broke Mark Spitz's single-Games record for most gold medals won. In addition, Phelps became the most decorated Olympian in history, having won 14 career gold medals, with six won in the 2000 Games in Athens.
E! Online said the athlete filmed a "spontaneous" cameo with the cast of "Entourage" in New York City this week, however, it is unclear when the episode will air.
The new season of the show kicks off Sept. 7.
Phelps has also agreed to be a presenter at the MTV Video Music Awards ceremony and will serve as host of the season premiere of "Saturday Night Live."
Celebrity Families of 2014 [PHOTOS]