James Harden Tosses (Fake?!) Birthday Cake Into Ocean During Yacht Party

James Harden's yacht birthday party literally takes the cake -- takes the cake and throws the whole 4-tier dessert overboard -- during the NBA star's wild bday celebration!

The Philadelphia Sixers guard was caught on video partying with his friends and family on a yacht this week in Marina Del Rey in California, celebrating his 33rd birthday.

Everyone was taking photos of Harden, dressed in all white, holding the large cake near the side rail of the yacht, when out of nowhere, he tossed the dessert overboard.

The party DJ was in complete shock ... saying on the mic, "Happy Birthday -- oh my God!"

But, was it real?! The cake's large, and while Harden is obviously a big, strong, world-class athlete, the cake seems awfully light! So, likely fake, but still super funny.

Thankfully, the fun didn't stop when the cake departed the boat ... Harden even showed off his "sexy" dance moves in front of the camera to Pretty Ricky's 'On The Hotline'.

There was also a cool ice sculpture of Harden aboard the yacht.

Harden and his guests celebrated the big 3-3 by sipping on Tequila Diamante, according to the party drink menu. They also enjoyed JH's new vino, J-Harden Wines.