Mr Kerry was joined by foreign ministers from the G7 group of nations who are holding talks in the city.
They laid wreaths at the memorial and observed a minute of silence.
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The ministers also visited the Bomb Dome, over which the A-bomb exploded, and the nearby Hiroshima museum, which tells the personal stories of people who died.
Mr Kerry wrote in the museum guestbook that it was "a stark, harsh, compelling reminder not only of our obligation to end the threat of nuclear weapons, but to rededicate all our effort to avoid war itself".
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