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.
X Factor judge Cowell also praised runner-up Grace Davies, describing her as "really an outstanding, outstanding artist".
The finalists performed several original songs during their appearances on X Factor, marking a change in the show's approach this year.
In the ratings battle, however, the first part of the X Factor final - which was shown on Saturday night - lost out to Strictly Come Dancing.
The BBC dance contest's quarter final had an average audience of 9.7 million, while the first night of the X Factor final averaged 4.4 million viewers. The two shows overlapped for just over an hour on Saturday.
It was a similar story on Sunday night, when they overlapped for 40 minutes, with an average audience of 5.2 million for the second part of the X Factor final and 10.3 million for Strictly.
Both Rak-Su and Davies previously had songs played by BBC Introducing before auditioning for the X Factor.
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