The International Atomic Energy Agency has revealed its Director General Yukiya Amano died last Thursday.
He was 72.
With his background in nuclear disarmament and non-proliferation, the Japanese diplomat had been heading the UN agency since 2009.
He was preparing to step down in March 2020 due to his deteriorating health.
During his term, he was involved with the cleanup of the 2011 Fukushima nuclear disaster.
Amano also helped negotiate the 2015 Iran nuclear deal.
While nothing has been decided about the next director general, flags at the IAEA's headquarters in Vienna have been lowered to half-mast.