The singer-songwriter tops the bill on August 19 as the event celebrates its tenth anniversary.
"Green Man is downright delighted to unveil the wonderful Feist as his first fantastic headliner for summer 2012!" the festival announced.
"What better way to end Green Man's tenth birthday bash than in the company of one of the most beguiling voices in modern music? Heavenly…"
Other acts confirmed for this summer's festival included The Walkmen, Jonathan Richman, The Felice Brothers, Yann Tiersen French, Ghost Poet and more.
Green Man takes place at Glanusk Park, Wales from August 17-19.
Tickets are on sale now and are priced at £145 for adults, £125 for students, £75 for those aged 13-7 and £5 for those aged 5-12.
Festival-goers can camp at Glanusk Park from three days before the event for an additional £40.
Feist has also been confirmed for Norway's Øya festival and Coachella in California.
Watch the Feist 'How Come You Never Go There' music video below: