Argentine diplomat Rafael Mariano Grossi assumed the position of director general of the International Atomic Energy Agency on Tuesday.
The appointment of Grossi was confirmed at a special session of the UN nuclear watchdog agency held one day earlier.
He succeeds Yukiya Amano from Japan, who died in July. The agency's board elected Grossi as his successor in October.
The 58-year-old diplomat is a nuclear nonproliferation expert and had served as Argentina's ambassador to the IAEA. He will become the first IAEA director general from South America.
In a speech on Monday, Grossi expressed his resolve to tackle nuclear nonproliferation and disarmament. Emphasizing the role of the IAEA, he said: "No one else can provide this credible assurance that nobody is diverting nuclear material to make nuclear weapons."
Grossi is likely to face many challenges during his four-year term. These include the 2015 deal between Iran and world powers on the nation's nuclear program. The United States has withdrawn from the pact and Iran is resuming its nuclear program in stages.
North Korea is likely to be another key focus. Pyongyang has hinted at conducting a fresh nuclear test as denuclearization talks with the US continue to stall.