After partying at her Beverly Hills 30th birthday bash on September 21, Nicole Richie has taken the celebration south of the border.

PHOTOS: Nicole's wild to mild transformation

The married mom of two enjoyed some fun the sun with fruity cocktails and close pals including Samantha Ronson (who also sported a two-piece) in Mexico Friday.

PHOTOS: Nicole and Joel's most romantic moments

Clad in a tiny black bikini, the birthday girl flaunted her healthy figure but showed no sign of vegging out on the massive three-tier birthday cake at her A-list party two days prior, where she gathered a bevy of family and friends like her hubby Joel Madden, Ronson, Mary-Kate Olsen, Benji Madden and dad Lionel Richie at a deluxe suite at the Beverly Wilshire.

NEWS: Nicole's star-studded b-day bash -- all the details

"I can't believe the outpour of birthday love I am getting on twitter today!" Richie wrote on her Twitter account September 21. "From the bottom of my heart (& the top), THANK YOU SO MUCH!"

Get more Us! Follow us on Twitter, Friend us on Facebook, Subscribe to Us Weekly