Another HUGE addition to the upcoming Michael Jackson tribute concert ... TMZ has learned Jamie Foxx has signed on and will perform LIVE in Wales at the October show.

Foxx joins Christina Aguilera, Cee Lo, Smokey Robinson and others who are set to perform at the Oct. 8 show.

Sources connected to the show tell TMZ ... show organizers are pumped up about Beyonce's performance -- and insist she did NOT pull out, despite a report to the contrary.

We're told B is recording a "unique, never before seen performance" that will be played at the show ... just as she committed to do.