Mr. Obama flew into the Iwakuni US base nearby, after leaving the G7 summit.
He said his visit was "a testament to how even the most painful of divides can be bridged." But he also says he will not be apologizing for the attack.
At least 140,000 people died in Hiroshima on 6 August 1945, in what was the world's first nuclear bombing.
Two days later a second nuclear bomb was dropped on Nagasaki, killing another 74,000.
'Best of Friends.'
Television footage has shown Mr. Obama arriving and entering the Hiroshima Peace Memorial Museum.
Mr. Obama earlier told service personnel at the Iwakuni Marine Corp base, some 40km (25 miles) from Hiroshima: "This is an opportunity to honor the memory of all who were lost during World War Two.
"It's a chance to reaffirm our commitment to pursuing the peace and security of a [world] where nuclear weapons would no longer be necessary."
Mr. Obama praised the US-Japan alliance as "one of the strongest in the world", with his visit showing how "two nations, former adversaries, cannot just become partners, but become the best of friends".