Car after car pulled up to the lovebirds' £13million Bel-Air mansion, as a host of Jennifer and Justin's celebrity friends including Don Cheadle, Jason Bateman, Ellen Degeneres and her wife Portia De Rossi attended the star-studded party.
The guests happily chatted away on the outdoor candle-lit patio, as they tucked into hors d'oeuvres and mingled with each other.
It was speculated that the party would be a cause for a double celebration, with Jennifer and Justin reportedly planning to tie the knot as Justin had proposed on his birthday last year.
The celebration, however, turned out to be a low-key birthday party and the guests dressed casually for the occasion.
The Hollywood power couple seemed happy to be hosting an intimate party with their nearest and dearest, as they have been away from home for some time. The Friends star was recently in New York City working on her latest project, Squirrels to the Nuts, while her fiancé was also away filming for The Leftovers.
Fans of the couple are waiting to see them become husband and wife, but Jennifer has previously revealed that she is in no rush to walk down the aisle and quashed rumours that they had called off the wedding.
"We just want to do it when it's perfect, and we're not rushed, and no one is rushing from a job or rushing to a job," she said.
"We have yet to set any date. There have been no cancelled weddings. There have been no postponed weddings. There have been no arguments about where to get married. Just clearing all that up."