Pool party in Sin City!

Katy Perry, buddy Rihanna and a gaggle of other pals rocked sexy swimsuits at the Hard Rock Hotel Beach Club in Las Vegas on Saturday. The occasion: Perry's bachelorette weekend.

The "Teenage Dream" singer will tie the knot with British comic beau Russell Brand this fall. See more hot shots of Katy and Rihanna in their bikinis.

Perry, 25, showed off her abs in a white bikini; Rihanna, 22, wore a sparkly gold one-piece. After checking into the Hard Rock's Casino Tower, the lively revelers got it started at a private cabana -- with drinks, nibbles and naughty bachelorette-themed toys, a rep tells

Later in the night, the group checked out Cirque du Soleil show Ka at the MGM Grand -- and feasted at AGO.

"Russell's so sensitive and sweet," Perry gushed to Rolling Stone of Brand, 35. "The Russell that people know from the media is not him at all"