Kian Egan, Nicky Byrne, Mark Feehily and Shane Filan are planning to release new music and will also go on tour next year, reportedly playing stadiums and arenas.
The group made the announcement in a video on social media which showed them all walking up and sitting on stools facing the camera.
"We'll be seeing you soon," they announced after reportedly signing a deal with Universal.
The reunion marks the 20th anniversary of the band starting in 1998.
But they will return without former member Brian McFadden, who left in 2004 and this week signed up to TV's Dancing On Ice.
Filan told The Sun: "We genuinely missed each other and missed the buzz of performing on stage together to all our fans.
"We still have a lot that we want to achieve as a band and we want to bring our new music to fans all over the world."
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