The cute couple headed to the desert destination for a two-day celebration of their engaged friends' separate bachelor and bachelorette parties.
The festivities kicked off Friday night when the two parties started at separate restaurants for dinner. Biel and the gals hit Fix at the Bellagio while Timberlake and the boys dined at Yellowtail (also at the Bellagio).
Funnily enough, JT's ex Britney Spears was just down the hall at the same hotel. The 31-year-old pop star was at Hyde Bellagio and the "O" Theatre for Cirque du Soleil's One Night for ONE DROP charity event, where other stars, like Michael Phelps, Shania Twain and Pamela Anderson, were helping raise money for global water-related issues.
After narrowly crossing paths, Biel continued the bachelorette partying on Saturday night while an under-the-weather Timberlake rested inside his hotel room. The 31-year-old actress and her girl group hit 1 OAK Nightclub at the Mirage where the party continued into the early morning.
Meanwhile, Spears dined with boyfriend David Lucado at Meatball Spot in Vegas' Town Square Saturday night.