Barr, 55, who joined the then Boase Massimi Pollitt in 1972, is planning to pursue personal interests.
Barr said: "After 31 years with Boase Massimi Pollitt, BMP Universe, BMP Davidson Pearce, BMP DDB Needham, BMP DDB and finally DDB London, I've decided the time has come to settle down. So I'm going to do other things that interest me but, sadly, don't earn any money."
He added that his decision was not connected to DDB's ongoing hunt for a high-profile senior management figure for the London office.
Chris Cowpe, Barr's fellow joint chief executive, is to remain in his role.
James Best, the group chairman, said: "When someone has been at an agency for over 30 years and done as much for it as Ross has for us, you can understand why he might like to call it a day and do something different with his life."
Barr and Cowpe joined the agency on the same day in 1972. Barr became the joint planning director in 1985 and the joint managing director in 1990. He took the top job at the agency in 1999 when Chris Powell, who had been the chief executive since 1986, took a more backseat role as the chairman.
In September last year, Powell announced his decision to stand down as part of a radical restructure of senior management to find fresh blood to rejuvenate the agency.
DDB London, which dropped its BMP name at the start of the year, has increased its pan-regional clients but not performed well domestically.