The metal band, who released "The Hunter" in 2011, were revealed by organisers of the festival after a contest on Twitter to get one message retweeted 5,000 times.
Once that target was met, the name was revealed by Andy Copping. Tweeting about Mastodon, he wrote: "5,000 has been reached! Mastodon are confirmed for the main stage Saturday under Maiden, QOTSA, Motorhead, Alice In Chains."
Mastodon join Iron Maiden, Rammstein and Slipknot on the line up for Download 2013. Queens Of The Stone Age, Bullet For My Valentine, 30 Seconds To Mars, Motorhead and The Gaslight Anthem also appear on the bill.
Download Festival will take place from June 14-16 2013 at Donington Park. The festival was headlined by Metallica, Black Sabbath and The Prodigy in 2012.
Meanwhile, it was recently revealed that Mastodon will be writing songs for the forthcoming sequel to Pixar movie "Monsters Inc".