The pair have been added to the bill alongside Felix Da Housecat and Totally Enormous Extinct Dinosaurs for the event, which runs between July 12 to July 15 in Serbia.
Plan B - real name Ben Drew - will perform tracks from his 2010 album The Defamation of Strickland Banks as well as material from his new LP ill Manors, while Avicii will play his chart hits including 'Seek Bromance', 'Levels' and 'Fade into Darkness'.
They join a long list of already-confirmed acts, including Guns N' Roses, New Order and Gossip.
Tickets, priced between £89 and £220, are on sale now. Those who purchase four tickets between March 19 and April 15 will be given a fifth ticket for free.
Last year's Exit Festival included performances from Pulp, Arcade Fire and Beirut.
Watch previous highlights from Exit Festival below: