The boyband will play nine extra shows on the tour after experiencing "phenomenal demand" for their initial run of shows.
The newly-confirmed gigs will take place throughout June, with extra dates added in London, Cardiff and Newcastle.
The tour will be in support of Westlife's upcoming Greatest Hits collection and will be the final chance for fans to catch the group live after they recently confirmed plans to split after 14 years together.
Speaking about the shows, the band have previously stated: "We're incredibly proud of what we've achieved and are looking forward to releasing our Greatest Hits album and performing them live on the tour as a special thank you to our fans who have supported us throughout our career."
Tickets for the extra dates will go on general sale from 9am on October 28.
Westlife's Greatest Hits will be released in the UK on November 21 and will be preceded by their new single 'Lighthouse' on November 13.
> Westlife to bow out in ITV1 television special
The extra dates for Westlife's farewell tour are as follows:
- June 5 - Nottingham, Capital FM Arena
- June 7 - London, O2 Arena
- June 8 - Birmingham, LG Arena
- June 9 - Manchester, Evening News Arena
- June 11 - Cardiff, Motorpoint Arena
- June 12 - Sheffield, Motorpoint Arena
- June 13 - Liverpool, Echo Arena
- June 15 - Newcastle, Metro Radio Arena
- June 16 - Glasgow, SECC
Watch the music video for Westlife's new single 'Lighthouse' below: