Sunao Tsuboi, an engineering student at a university in Hiroshima, was making his way across the city’s Miyuki Bridge en route to a morning class when, in a sudden, terrifying moment on Aug. 6, 1945, blazing light flooded his surroundings. A blast threw him more than 30 feet, leaving him unconscious. When he awoke, the light was gone, subsumed by an awful pall. “I know now,” he told the British Daily Mirror decades later, “I was under the mushroom cloud.”

Ms. Tsuboi emerged from the U.S. atomic attack on Hiroshima with burns across his entire body. “My ears were hanging off,” he recounted. “I saw tens of thousands of bodies everywhere, all burned and dead. I saw such terrible things. One girl had her right eye hanging from its socket, next to her jaw. A woman was trying to force her own intestines back into her body. An old man’s lung was sticking through his chest.”

Convinced of his own impending death, Mr. Tsuboi used a pebble to scrawl the words “Tsuboi died here,” a marker for friends who might come searching for his remains. When he died Oct. 24 at 96, he had outlived by more than 76 years his expectation that day, when the world first observed the horror of nuclear war and when Mr. Tsuboi was set on his path as an international advocate for disarmament.

Mr. Tsuboi eventually reached a military hospital, where he said he again lost consciousness. Only weeks later, after regaining his orientation, did he learn that World War II was over and that Japan had surrendered after the United States dropped a second atomic bomb on the city of Nagasaki.

By the end of 1945, an estimated 140,000 people had died in Hiroshima and 74,000 more had perished in Nagasaki from the effects of the blasts, according to the International Campaign to Abolish Nuclear Weapons, the Nobel Peace Prize-winning organization in Geneva.

“I don’t know why I survived and lived this long,” Mr. Tsuboi said in 2015, according to the Agence France-Presse. “The more I think about it . . . the more painful it becomes to recall.”

Sunao Tsuboi was born in Ondo, on Kurahashi Island, toward the southern tip of Japan, on May 5, 1925. He was 16 when Japan attacked Pearl Harbor, the event that precipitated U.S. entry into World War II, and 20 years old, a student at what is now Hiroshima University, when the United States ended the war with the two atomic attacks.

Although he did not realize it at the time, Mr. Tsuboi was less than one mile from ground zero of the blast at Hiroshima. His clothes were burned off, and he recalled running, naked, until he could no longer go on.

Against all odds, his mother located him at the field hospital where he was taken for treatment.

“She had a strong love for her son,” Mr. Tsuboi told the Honolulu Star-Bulletin in 2007. “She, in a loud voice, shouted my name, ‘Sunao.’ It was miraculous.”

Mr. Tsuboi embarked on a long recovery, crawling before he could once again walk. With the scars from his burns, he was easily recognizable as one of the hibakusha — the Japanese word for survivors of the atomic blasts — and said he encountered discrimination in both his professional and personal lives because of his past.

The parents of his future wife, Suzuko, at first opposed their marriage because they worried that Mr. Tsuboi, who, like other survivors of the atomic attacks, suffered from the effects of radiation, would leave their daughter a widow. Discouraged from marrying, the young couple took sleeping pills in a double-suicide attempt. They survived and ultimately wed.

The couple had three children and seven grandchildren, but a complete list of survivors was not immediately available.

Mr. Tsuboi became a math teacher and later a junior high school principal. With the passage of time, he became increasingly involved in disarmament advocacy, working closely with Nihon Hidankyo, an organization of survivors of the Hiroshima and Nagasaki attacks.

Koichiro Maeda, a representative of the group’s Hiroshima branch, wrote in an email that Mr. Tsuboi died at a hospital in Hiroshima of an abnormal cardiac rhythm caused by anemia. Mr. Tsuboi also suffered several bouts of cancer, which he attributed to the radiation from the blast.

Mr. Tsuboi traveled around the world speaking about the catastrophic effects of nuclear war. He had perhaps his most significant audience in 2016, when President Barack Obama became the first sitting U.S. president to visit Hiroshima. They clasped hands, Mr. Tsuboi recalled, in a meeting where the emotions, he said, required no interpreter.

“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.”