Sunao Tsuboi, a survivor of the Hiroshima atomic bombing, died at 96, according to the BBC.
He died at a hospital in Hiroshima of an abnormal cardiac rhythm caused by anaemia.
He also suffered several bouts of cancer, which he attributed to the radiation from the blast.
Sunao Tsuboi was a 20 years old university student when he was caught in the blast on 6 August 1945, suffering burns all over his body.
He expected his immediate death. In the end, he had outlived that expectation by more than 76 years.
“I don’t know why I survived and lived this long,” said Mr Tsuboi in 2015, according to the Agence France-Presse.
“The more I think about it . . . the more painful it becomes to recall,” he added.
The explosion killed about 140,000 people in Hiroshima and another 74,000 in Nagasaki.
Mr Tsuboi dedicated his life to campaigning to eradicate nuclear arms.
He travelled around the world speaking about the catastrophic effects of nuclear war.
“At some point in the next century, I won’t be here,” Mr Tsuboi told the Agence France-Presse in 1999. “We really need to let the next generation take over the mission to tell the horrible story and urge the world not to repeat the same mistake.”
Mr Tsuboi taught mathematics at schools in Japan, telling youngsters about his experiences during the war.
“Never give up” was his advice for anti-nuclear campaigners.