The Watford-based group beat Grace Davies in the final of the ITV singing competition - the first boy band to win since the show started in 2004.
Rak-Su thanked viewers for voting for them, while their mentor Simon Cowell hailed them as "stars".
Proceeds from Rak-Su's winners' single Dimelo will go to children's hospice charities Together For Short Lives and Shooting Star Chase.
The track, a duet with Wyclef Jean and Naughty Boy, was first performed during Saturday's show.
