The Take That frontman will join the final five contestants on tonight's latest live show for a special one-off performance of Greatest Day.
Welsh Opera star Katherine Jenkins will perform on Tuesday's live semi-final, while Superstar judge Mel C will take to the stage herself on Wednesday for the final.
Five singers and potential Jesuses still remain in the competition, with one being crowned winner of the show on Wednesday night.
Other special guests who have appeared on the series have included the Scissor Sisters and the Kaiser Chiefs.
Superstar airs tonight on ITV1 from 9PM.