The singer will play the concert on the Sunday along with previously announced acts JLS, Wretch 32 and The Saturdays.
One Direction, Ed Sheeran and Emeli Sande will also play the Capital FM event on December 4 at London's O2 Arena.
However, the first batch of tickets for the show sold out in under an hour when they were released at 8am this morning.
It was previously confirmed that Rihanna, Olly Murs and The Wanted will play the first day of the event on December 3.
Pixie Lott, Calvin Harris and Jason Derulo also have slots on the Saturday, with the initial tickets selling out within minutes.
Jessie J recently revealed that she has worked with David Guetta on tracks for her second studio album.