Khloe Kardashian rang in the big 3-0 with beau French Montana by her side.

The reality star and the rapper made their first public appearance together at Kardashian's birthday party in Sin City on Friday night, and he "never left her side," a witness told us.

Kardashian, who filed for divorce from hubby Lamar Odom in December, and French, 29, started dating this spring.

The pair started their night with dinner at TAO -- joined by pals Malika Haqq, Khadijah Haqq McCray and singer Miguel and his gilfriend Nazanin Mandi -- then headed to TAO Nightclub where they camped out at a VIP table.

Kardashian also threw some support her boyfriend's way at the club: When DJ Eric D-Lux spun some of French's tunes, she "sang along as partygoers went wild," the spy dished.