Simpson will join headliners Fat Boy Slim, Eliza Doolittle and Klaxons, who were confirmed to appear back in March.
The event is the UK's only surf, skate and music festival and will take place at Fistral Beach and Watergate Bay in Newquay, Cornwall between August 10 and August 14.
The Relentless Boardmasters Festival will also include a bigger dance stage after the area proved popular with festival-goers in previous years.
The new stage, named TUACA Point, will play host to established dance acts including Krafty Kuts, Jaguar Skills and Breakage, along with TUACA-branded cocktails to keep in theme with the beach-based festival.
The music event will be one of the first opportunities for Simpson's fans to hear songs from his debut solo album Young Pilgrim, which will be released on August 15.
Tickets for the Relentless Boardmasters Festival are on sale now.
Watch the music video for Charlie Simpson's new single 'Parachutes' below: