The 'Video Games' singer will perform the one-off gig at London's Roundhouse on September 25.
Pianist Robert Glasper has also joined the growing lineup on September 23.
Conor Maynard will also support JLS for their gig at the venue on September 6.
The pair join the likes of Jack White, Ed Sheeran, Noel Gallagher, Mumford & Sons, Deadmau5, One Direction and Jessie J for the festival, which takes place throughout September.
Each gig will be streamed live via iTunes, with tickets only available by winning them through the festival's website.
Lana Del Rey's original version of Cheryl Cole's 'Ghetto Baby' was leaked online earlier this week.
The lineup for the iTunes Festival so far is as follows:
Usher (September 1)
Ed Sheeran, Charli XCX (2)
Olly Murs (3)
Emeli Sande (5)
Plan B (5)
JLS, Conor Maynard (6)
Jack White (8)
Norah Jones (10)
Noel Gallagher's High Flying Birds, The Soundtrack Of Our Lives (12)
Labrinth, Josh Kumra (14)
David Guetta, Calvin Harris (15)
Rebecca Ferguson (16)
Example, DJ Fresh, Hadouken! (17)
Andrea Bocelli (18)
Matchbox Twenty (19)
One Direction (20)
Jessie J, Lonsdale Boys Club (21)
Biffy Clyro, Frightened Rabbit (22)
Robert Glasper (23)
Mumford & Sons (24)
Lana Del Rey (25)