The Take That frontman will join the final five contestants of the ITV1 talent show for a live performance of 'Greatest Day'.
The five hopefuls will sing while Barlow will accompany them on piano on Monday, July 23 at 9pm.
Barlow's appearance will see him reunite with Lord Andrew Lloyd Webber, with whom he co-wrote the Diamond Jubilee track 'Sing' earlier this year.
Katherine Jenkins will perform during Tuesday's live semi-final episode.
Superstar judge Mel C will then take to the stage on Wednesday's grand final (July 25).
Jon Moses and Jeff Anderson were eliminated from the latest episode of Superstar on Friday.
The search to find a Jesus to appear in Lloyd Webber's new production of Jesus Christ Superstar continues tomorrow at 9pm on ITV1.