Brody Jenner and his girlfriend Avril Lavigne celebrated his 28th birthday at The Bank Nightclub at Bellagio Resort & Casino. We're told Brody had just flown in from Montecito where he attended step-sister Kim Kardashian's wedding to Kris Humphries.

Bruce Jenner's sexy son didn't pose with his ladylove, but her name could be visibly seen on his forearm. That's true love!

Once arriving in Sin City, Brody Tweeted: "The ship has arrived! VEGAS TIME BABY…Gonna be a great one."

Brody and Avril dined at Yellowtail Japanese Restaurant & Lounge ... munching on signature dishes like Big Eye Tuna Pizza, Rock Shrimp and Lobster Carpaccio by Executive Chef, Akira Back. The sexy duo, and a group of friends, then headed to The Bank ... leaving around 4:00AM.

Sounds like a sinfully delicious night all around!