The Welsh band, who won won the BBC Introducing Award in 2014, played a small venie at the festival a few yeara ago but are now a bill-topping act nationwide.
Indoe band The Crookes have also been announced for the festival, to play oin the Devonshire Green stage on the Saturday, while Blessa will play the Queens Social Club on the Sunday.
All three acts join a line-up that already includes Firday night headliner Dizzee Rascal, Dawn Penn, Goldie, Young Fathers, Toddla, Norman Jay, Amy Becker, Mumdance, Field Music, Hinds, Gwenno, C Duncan, Randall, Meilyr Jones and more.
Tramlines runs from July 22-24.
Tickets and information about the festival i available online at www.tramlines.org.uk