'Feel The Love' stars Rudimental, Disclosure, dan le sac and Scroobius Pip have also been added to the bill.
Florence + the Machine, New Order and Stevie Wonder have already been confirmed as festival headliners.
Nash caused a stir earlier this month after new song 'Under-Estimate The Girl' premiered online, marking a significant change of musical direction for the 'Foundations' singer.
Meanwhile, Stooshe recently scored their second top five hit with 'Black Heart'.
Other new additions to Bestival include Clement Marfo & The Frontline, Summer Camp, The Correspondents, James McCartney and The Coronas.
Beans on Toast, Two Jackals, Lucky Elephant, Sleepy Sun and Gush will also perform.
Bestival takes place at Robin Hill Country Park on the Isle of Wight between September 6 and 9.
Watch Kate Nash's video for 'Under-Estimate The Girl' below: