Host Dermot O'Leary confirmed at the end of tonight's (December 2) show that the singers will appear in front of 12,000 fans at Manchester Central next weekend.
The pair join fellow guests One Direction and Emeli Sandé, who were previously announced to be appearing on the show.
It has also been confirmed that the finalists will take to the stage to perform duets with their respective mentors.
James Arthur and Jahmene Douglas will perform with Nicole Scherzinger, while Christopher Maloney will sing with Gary Barlow.
Union J were voted off The X Factor earlier tonight, narrowly missing out on a place in the final.
The X Factor final starts next Saturday (December 8) at 8pm on ITV1.
Watch Rihanna performing 'Diamonds' on last weekend's X Factor below:
Photo gallery - The X Factor semi-final results: