The rapper will take to the Main Stage at both sites across the weekend of August 23-25, 2013.
He is joined on the lineup by Mercury Music Prize winners Alt-J , who will headline the NME/ Radio 1 tent.
Deftones will also perform on the Main Stage, while Sub Focus will headline the BBC Radio 1 Dance Stage. Boy Better Know will headline the new BBC Radio 1 Xtra Stage.
It was previously announced that three new stages would be present at next year's event.
Festival Republic boss Melvin Benn said: "I've got quite a strong buzz and feeling in myself in the changes that I'm making there.
"I'm still formulating the ideas. There will be quite a bit more dance music, quite a bit more other bits and pieces."
Tickets for the Reading and Leeds Festivals 2013 are on sale now, sold at 2012's prices until December 31. Fans can guarantee a place at the festival with a £50 deposit, as long as the full balance is paid before March 31, 2013.
Watch Eminem performing at 2010's T in the Park below (warning: strong language):