Arsala Rahmani was a former Taliban minister and a key member of Afghanistan's High Peace Council, which leads Afghan efforts to negotiate a peace deal with the Taliban.
Correspondents say his death is a major blow to President Hamid Karzai as Mr Rahmani was a key figure in reaching out to Taliban commanders.
Last year the chief of the peace council was killed in a suicide attack.
Burhannudin Rabbani was killed by a bomb hidden in the turban of a suicide attacker posing as a Taliban peace envoy last September.
His US-educated son, Salahuddin Rabbani, was appointed to replace him last month.