Victoria Beckham's 50th birthday party brought the flavor to Mayfair, London ... 'cause all five of the Spice Girls reunited for the milestone birthday bash!

Mel B, Melanie C, Emma Bunton and Geri Halliwell all pulled off for the stylish party to celebrate their longtime pal ... and, check out the pics. They're dressed to impress.

Of course, Posh herself was there though it seemed she shied away from the camera a bit ... focusing on her guests and hangin' firmly with 12-year-old Harper all night. She's still using crutches BTW ... after breaking her foot in a gym accident.

BTW ... all the Beckham kids made it out for their mom's bday -- with her boys Brooklyn, Romeo and Cruz pulling up as one big squad and husband David Beckham also attending.

And, the A-List only begins with the Spice Girls and the Beckhams ... because some of the biggest stars on the planet also showed up for VB's big day.

Tom Cruise looked dapper in a tuxedo on the big night, and Gordon Ramsey arrived with wife Tana in tow.

Among the other bold-faced names ... Guy Ritchie, Jason Statham, Victoria's Secret Angel Rosie Huntington-Whiteley, Marc Anthony and his new wife Nadia Ferreira, Eva Longoria, and a whole lot more.

It's a good sign the Beckhams are still throwing parties ... like we told you, investigative biographer, Tom Bower's promising a tell-all book full of stories about David's rumored affairs -- a glossed-over topic in their Netflix doc.

Seems the Beckham clan isn't sweating the release though ... and, their longtime friends totally have their back.