Brother, the group's fifth number one album, first reached the top spot a fortnight ago but was beaten by Lady GaGa's The Fame Monster last week.
However, it has now reclaimed the crown after a 60% increase in sales week-on-week.
Speaking to Radio 1, band member Mikey Graham said: "It's absolutely amazing that we've had a number one album again with Brother... We really want to thank all our fans for the support they've given us not only in the last 17 years, but particularly in the last six months [since the death of Stephen Gately]. Thank you very, very much from me and all the boys."
There were two new entries in the top ten, with Laura Marling's I Speak Because I Can charting at number four and Goldfrapp's Head First going straight in at number six. Justin Bieber re-entered the top ten, climbing 30 places to number three with My Worlds.
However, Gorillaz fell eight places to number 12 with Plastic Beach, while Amy Macdonald's A Curious Thing dropped to number 15. The first compilation of songs from Glee also slipped five places, charting at number 11.
The top ten singles in full (click where possible for our reviews):
1. (3) Boyzone: 'Brother'
2. (1) Lady GaGa: 'The Fame Monster'
3. (33) Justin Bieber: 'My Worlds'
4. (-) Laura Marling: 'I Speak Because I Can'
5. (2) 'Glee' Cast: 'Glee: The Music, Volume II'
6. (-) Goldfrapp: 'Head First'
7. (7) Paolo Nutini: 'Sunny Side Up'
8. (5) Alicia Keys: 'The Element Of Freedom'
9. (8) Florence and the Machine: 'Lungs'
10. (9) Mumford & Sons: 'Sigh No More'
Source: Official Charts Company
> Click here for this week's top ten singles