Organizers say the band, whose hits include "I Gotta Feeling," has withdrawn due to "unavoidable circumstances."
Christina Aguilera, Smokey Robinson, Gladys Knight, Leona Lewis and Cee Lo Green are on the bill for Saturday's "Michael Forever" show at Cardiff's Millennium Stadium.
Chris Hunt, chief executive of event organizer Global Live Events, said Wednesday that the concert would be a "fantastic evening," despite the absence.
The show is not affiliated with Jackson's estate, but his mother Katherine and other relatives are scheduled to attend.
Organizers have rescinded an offer to the band Kiss over singer Gene Simmons' past criticism of Jackson.