On the day of the bombing, Mr. Taniguchi, then 16, was delivering mail on his bicycle in the northern corner of the city, just over a mile from ground zero.
When the bomb detonated overhead, the force of the explosion tossed him into the air, and the heat it radiated melted his cotton shirt and seared the skin off his back and one arm.
Three months later, he was taken to a navy hospital, where he lay on his stomach for nearly two years. Bedsores formed on his chest and left permanent scars.
He spent a total of more than three and a half years in the hospital after the bombing. Sometimes he was in so much pain, he said, that he would scream to the nurses, “Kill me, kill me!”
In 1946, United States forces filmed his treatment. That footage was shared across the world, and Mr. Taniguchi became known as “the boy with a red back.” When giving speeches calling for the abolition of nuclear weapons, he would sometimes show pictures of his burns to illustrate the horrible suffering that resulted from the bombings.
A decade after the war, when Mr. Taniguchi had learned to sit up, stand and walk again, he joined a youth group for survivors and began working as an activist. He spoke at memorial ceremonies in Hiroshima and Nagasaki and took part in antinuclear marches in New York. He continued to speak out until close to death, traveling last year to Malaysia to deliver a speech against nuclear proliferation.
Mr. Taniguchi was born on Jan. 26, 1929, in Fukuoka, on the southern island of Kyushu. According to the Nagasaki Shimbun, a local newspaper, his mother died when he was just 18 months old and his father, a train operator, was sent to Manchuria during the war. With his elder sister and brother, Mr. Taniguchi went to live in Nagasaki with their mother’s parents.
After graduating from middle school, he went to work at the post office.
Mr. Taniguchi’s wife, Eiko, died last year. He is survived by his daughter, Sumie Terasaka, and his son, Hideo, as well as four grandchildren and two great-grandchildren.
In 2006, Mr. Taniguchi was appointed chairman of the Nagasaki Atomic Bomb Survivors Council, and in 2010 he gave a speech at the United Nations in New York during a meeting to consider a nonproliferation treaty.
A month before he died, the United Nations adopted the Treaty on the Prohibition of Nuclear Weapons.
“He played a tremendous role,” said Terumi Tanaka, secretary general of the Japan Confederation of A- and H-Bomb Sufferers Organizations. “But unless all countries sign the treaty, there is no guarantee that nuclear weapons will disappear.”
He added: “I wanted him to keep working together to achieve our complete goal. He staked his whole life on this movement.”
Every year on the anniversary of the Nagasaki bombing, as well as any time a country conducted a nuclear test, he would attend a sit-in at the Peace Park in that city. According to the Nagasaki Shimbun, he appeared at 396 protests.
At the memorial service in Nagasaki on the 70th anniversary of the bombing, Mr. Taniguchi criticized the government of Prime Minister Shinzo Abe for pushing through security bills that allow Japanese troops to engage in overseas combat missions. He said the bills could lead Japan into war again.
“I am determined to keep telling the reality of nuclear war as one of the living witnesses,” Mr. Taniguchi said, “to realize a world without wars and nuclear weapons as long as I live.”