26 May 2016 Last updated at 12:00 BST
The Hiroshima bombing on 6 August 1945 killed 140,000 people.
Along with a second bombing on Nagasaki, it led to Japan's surrender and the end of World War Two.
BBC News looks back at the events of August 1945 and the years that followed.
President Barack Obama will become the first sitting US President to visit Hiroshima, the site where an atomic bomb was dropped in 1945.