Grossi, 58, is currently Argentina's ambassador to the International Atomic Energy Agency and a veteran of nuclear diplomacy. He was in a close-fought race against Romania's Cornel Feruta, formerly Amano's right-hand man and currently the IAEA's acting director-general.
Previous votes by countries on the IAEA's 35-nation Board of Governors had failed to produce a clear winner but eliminated two other candidates. On Tuesday Grossi obtained the two-thirds of votes required, with 24 countries supporting him, 10 backing Feruta and one abstaining, diplomats said.
Tuesday's vote paves the way for Grossi to become the next director-general but must still be signed off on by the board, which aims to approve the winner by consensus, and by a larger meeting of all IAEA member states, both of which are considered a formality.