Guitarist David Gilmour, drummer Nick Mason, bass player Roger Waters and keyboard player Richard Wright have not performed on stage together since 1981.
The group, which achieved major success with their 1973 album "Dark Side Of The Moon," will join musical acts including Elton John, Madonna, Paul McCartney and Coldplay at the anti-poverty concert in Hyde Park on July 2.
"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," Gilmour said.
"It's crazy that America gives such a paltry percentage of its GNP to the starving nations."
Waters, the group's founder, split with the rest of the band after a falling-out in the 1980s.
"Any squabbles Roger and the band have had in the past are so petty in this context, and if re-forming for this concert will help focus attention then it's going to be worthwhile," Gilmour said.