The annoucement was made at a press conference at London's O2 arena which all five members attended.
The full line-up has not performed together since Ginger Spice Geri Halliwell quit in May 1998.
"I think for us it was about celebrating the past, enjoying each other and it's about our fans. It was the right time," said Halliwell.
The group will kick off their tour in Las Angeles in December, and will go on to perform in Las Vegas, New York, London, Koln, Madrid, Beijing, Hong Kong, Sydney, Cape Town and Beunos Aires.
Earlier this month Mel Chisholm, also known as Sporty Spice, told the BBC that any reunion would be for "a very short space of time... a final goodbye".
The singer added that she had resisted reforming the band in the past because "it was amazing, it was magical. We could never recreate it".
When asked at the conference why she had gone back on her word she said: "A girl is allowed to change her mind and also this is something that we have only seriously started this year really.
"I think really all of us have had our fears and doubts but we feel that the time is right."
All five women laughed and joked constantly with photographers and reporters, even jokingly booing one journalist.
Future plans
The tour is being put together by Simon Fuller, whose 19 company masterminded the group's global success more than a decade ago.
Under his guidance, the five-piece notched up a string of hits - including Wannabe and 2 Become 1 - while also capitalising on their fame with a stream of sponsorship deals.
Nicknames
Emma Bunton, Mel Brown, Mel Chisholm, Victoria Adams and Geri Halliwell quickly became household names - although they were better known as Baby, Scary, Sporty, Posh and Ginger. They sold more than 55 million records around the world, and even starred in a film, Spice World.
Halliwell quit in 1998 citing "differences", leaving them to complete a sold-out world tour as a foursome.
The group achieved their third consecutive Christmas number one later that year, and released a third album, Forever, in 2000.
Each member has since pursued solo careers with varying degrees of success, while Posh Spice has become better known as fashion icon Victoria Beckham.