Tens of thousands gathered to remember the more than 70,000 people who died instantly in the blast, or of the after-effects in the months and years after the bombing, which hit Nagasaki at 0732 IST.
Bells tolled as ageing survivors, relatives, government officials and foreign delegates observed a moment of silence at the time of detonation.
The bombing of Nagasaki came three days after the first-ever atomic blast at Hiroshima, which claimed about 140,000 lives in all. Hiroshima held its own remembrance ceremonies earlier this week.
The ceremony is held near the spot where the US military dropped its plutonium bomb, nicknamed "Fat Man", on August 9, 1945, in the final chapter of the war.
More From This Section
Historians have long been at odds over whether the twin attacks brought a speedier end to the war by forcing Japan's surrender and preventing many more casualties in a land invasion planned for later in the year.
Many atomic bomb survivors, known as "hibakusha" in Japanese, oppose both military and civilian use of nuclear power, pointing to the tens of thousands who were killed instantly in the blasts and the many more who later died from radiation sickness and cancer.