Roger Waters left Pink Floyd in the mid-1980s to pursue a solo career
Roger Waters will join band members Dave Gilmour, Nick Mason and Richard Wright for the show in Hyde Park.
Waters last performed on stage with them at London's Earls Court in 1981.
The Live 8 concert, organised by Bob Geldof and Midge Ure to help fight African poverty, is one of five taking place around the world on 2 July.
Pink Floyd join a star-studded line-up at Hyde Park which includes Madonna, REM, U2, Sir Paul McCartney, Sir Elton John, Coldplay and Robbie Williams.
"Like most people I want to do everything I can to persuade the G8 leaders to make huge commitments to the relief of poverty and increased aid to the third world," said Pink Floyd's Dave Gilmour.
"Any squabbles Roger and the band have had in the past are so petty in this context, and if reforming for this concert will help focus attention then it's got to be worthwhile," he added.
Legendary
Pink Floyd are viewed as being one of the biggest rock bands of all time.
The group released their first record in 1967, with founding member Syd Barrett on lead guitar, alongside Waters, Mason and Wright.
At the beginning of 1968, guitarist Dave Gilmour - a friend of the band - was brought in as a fifth member. Barrett left the group shortly afterwards.
The remaining four members achieved legendary status with albums such as 1973's The Dark Side Of The Moon, which stayed in the US album chart for more than a decade.
Fans have long yearned for a reunion of the classic Pink Floyd line-up of Waters, Gilmour, Mason and Wright, but it seemed unlikely because of a fall-out between founder member Waters and the rest of the group in the mid-1980s.
Gilmour, Mason and Wright continued to record and tour as Pink Floyd during the remainder of the 80s and into the 90s, releasing their last studio album - The Division Bell - in 1994.
Pink Floyd's "Dark Side of the Moon
Organizers of the upcoming London Live 8 concert have announced that legendary rock band Pink Floyd plans to perform on July 2. Guitarist David Gilmour, bassist Roger Waters, drummer Nick Mason, and keyboardist Richard Wright will play their first performance together since  playing at Earls Court, London in 1981.

The band will come together, along with other musical acts such as Coldplay, Elton John, and Paul McCartney for this event. Live 8 is a series of free concerts at each of the G8 nations organized by Bob Geldof. These concerts, set on the 20th anniversary of Geldof's Live Aid, have been planned to support the Make Poverty History campaign, which aims to reduce African poverty. 

This lineup of Pink Floyd has not played together for 24 years due to a conflict between David Gilmour and Roger Waters over leadership and creative influence of the band. However, it seems that they are willing to put their differences behind to support Live 8; "Like most people I want to do everything I can to persuade the G8 leaders to make huge commitments to the relief of poverty and increased aid to the third world," said David Gilmour and added, "Any squabbles Roger and the band have had in the past are so petty in this context, and if reforming for this concert will help focus attention then it's got to be worthwhile."