Savages, Loney Dear and Tom Williams & The Boat will also play at the event, which takes place in Wales's Brecon Beacons from August 17 to 19.
Mogwai, Feist and Van Morrison were previously announced as headliners.
Additions to the festival's Chai Wallahs marquee include Dizraeli and the Small Gods, Polaroid 85, Laid Blak, Heymoonshaker and dubstep band Coda.
They join acts including Stephen Malkmus & the Jicks, The Walkmen, Jonathan Richman, of Montreal, Michael Kiwanuka, Scritti Politti, Field Music, Toy and Willy Mason.
Metronomy released their breakthrough LP The English Riviera last April, which went on to be nominated for the Mercury Prize.
Watch Metronomy's 'Everything Goes My Way' music video below: