Last year the duo teamed up on the nostalgic pop banger "1999".
Having teased that her new album featured a new collaboration with Troye Sivan, the pair debuted the new single during a performance at the first annual Go West Fest - co-hosted and co-headlined by the collaborators.
The new single is set 100 years on from their last, in the year "2099".
Today BTS unveiled their new single "Dream Glow" that features Charli XCX.
Charli XCX has promised to release new music for "five months straight", and revealed that her new album is "kind of a continuation of Pop 2", and "more Charli than ever before".
"2099" hasn't got a release date yet. Charli XCX plays Reading and Leeds festivals in August. Find out more.