The quartet — Shaznay Lewis, Melanie Blatt, Natalie and Nicole Appleton — will make a guest appearance when the US boyband play five headline dates across the UK and Ireland next year.
"We are really looking forward to performing together as special guests of the Backstreet Boys," All Saints said in a statement.
The girls now
"The Backstreet Boys are a great band, so it's an honour. We're singing a lot of big songs like Never Ever and Pure Shores, which will be amazing. We can't wait!"
The Backstreet Boys are heading across the Atlantic in 2014 in support of their recent album, In a World Like This.
"Last time we were in the UK, we were living in a house together recording our album, which was incredible," said the chart-toppers — AJ McLean, Howie Dorough, Nick Carter, Kevin Richardson and Brian Littrell.
"Our last UK show were with NKOTB (New Kids on the Block) and we had a fantastic time — the reaction was out of this world. We can't wait to come back and blow you away with our own full headline show.
The girl group in 1995
"We're really excited for our UK fans to see us again as a five-piece. The original Backstreet's back!"
The shows will start at Birmingham's LG Arena on March 26 and finish at Manchester's Phones 4 U Arena on 5 April.
Inbetween, the two groups will perform at Dublin's O2 Arena (1 April), the Hydro in Glasgow (3 April) and the London O2 Arena (4 April). Tickets go on sale from 9am on 22 November.