"Today we remember all those people who lost their lives in Hiroshima. I hope humankind never sees such a day ever again," Modi tweeted.
"Let us all work together and further the cause of world peace and make the world a happier and more peaceful place in the years to come," he added.
On this day in 1945, an atom bomb was dropped by the US on Hiroshima in one of the final chapters of World War II. It had killed an estimated 140,000 by December that year.