Nas, Childish Gambino, and Iggy Azalea also among the more than 130 acts on the bill.
Both will headline Lollapalooza this year, which returns to Chicago's Grant Park Aug. 1-3, festival organizers announced Wednesday (March 26).
Em and the ATLiens join rock group Kings of Leon in leading the three-day event. Lollapalooza is one of over 40 live shows OutKast will play this year, to commemorate their 20-year anniversary as a hip hop duo.
The set will mark Em's first return to Lollapalooza since 2011. "He's on top of this game and he's doing a great show," Lollapalooza founder and Jane Addiction's frontman Perry Farrell told the Associated Press. "His live show is incorporating real well-made video and guest appearances."
This year's roster features more than 130 acts spanning different music genres. Other names on the 2014 bill include Nas, Iggy Azalea, Jhené Aiko, Chance the Rapper, Childish Gambino and "Royals" singer Lorde.
Single-day Lollapalooza passes are on sale now, three-day passes sold out on Tuesday (March 26).
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(Photos from left: Scott Legato/Getty Images, Kevin Winter/Getty Images)