Yamaguchi was in Hiroshima on a business trip on Aug. 6, 1945, when a U.S. B-29 dropped an atomic bomb on the city. He suffered serious burns to his upper body and spent the night in the city. He then returned to his hometown of Nagasaki just in time for the second attack, city officials said.
"As far as we know, he is the first one to be officially recognized as a survivor of atomic bombings in both Hiroshima and Nagasaki," Nagasaki city official Toshiro Miyamoto said. "It's such an unfortunate case, but it is possible that there are more people like him."
Certification qualifies survivors for government compensation -- including monthly allowances, free medical checkups and funeral costs
-- but Yamaguchi's compensation will not increase, Miyamoto said.
Japan is the only country to have suffered atomic bomb attacks. About 140,000 people were killed in Hiroshima and 70,000 in Nagasaki.
Yamaguchi is one of about 260,000 people who survived the attacks. Bombing survivors have developed various illnesses from radiation exposure, including cancer and liver illnesses.
Details of Yamaguchi's health problems were not released.
Thousands survivors continue to seek official recognition after the government rejected their eligibility for compensation. The government last year eased the requirements for being certified as a survivor, following criticism the rules were too strict and neglected many who had developed illnesses that doctors have linked to radiation.