Grossi, 58, became Argentina's ambassador to the Vienna-based agency in 2013 and was previously the IAEA's chief of cabinet under Amano. He said after his election that "the work of the agency is of the essence for international peace and security."
Argentine Foreign Minister Jorge Faurie has described him as "a career diplomat with more than 35 years of professional experience in the field of nonproliferation and disarmament." The IAEA said that Grossi beat Cornel Feruta of Romania by a 24-10 margin in Tuesday's vote at a closed session of the board of governors.
Feruta, who was the IAEA's chief coordinator under Amano and has been the acting director general since his death, was the last of three other candidates nominated for the job who was still in contention.
Also nominated were Lassina Zerbo of Burkina Faso, the executive secretary of the Comprehensive Nuclear-Test-Ban Treaty Organization, and Marta Ziakova, the head of Slovakia's nuclear regulatory authority.
The new director general is due to start his four-year term by Jan. 1 and will be the IAEA's sixth since the agency was founded in 1957. Amano, a Japanese diplomat who died in July at age 72, was extensively involved in negotiations over Iran's nuclear program and the cleanup of the 2011 Fukushima nuclear disaster in Japan.