The two-tier cake given to the swimmer read:
"Congratulations To The World's Most Decorated Olympian."
The world's best was having the time of his life while getting it in Vegas-style at the Encore Beach Club!
There were models in tiny bikinis everywhere, music, and his best friends in the world were also present... what more could he have asked for?
In celebration of the good times, Michael even retweeted a die hard fan:
RT @ccca02: @MichaelPhelps in 2004 you were my first crush I ever had on a boy and I was 8. I'm now 16 and still love you. so proud of all you've done<3
We're proud of you too! Check out the pictures from his retirement party in the gallery below and let us know what you think!