The International Atomic Energy Agency's board of governors selected Grossi as its new director general. The appointment still requires the formal approval of the IAEA's general conference, in which all of the nuclear watchdog's 171 member countries are represented.
Grossi is Argentina's ambassador to the Vienna-based IAEA. The IAEA said that he 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.