Grossi is a candidate along with Chile's Michelle Bachelet, Ecuador's Maria Fernanda Espinosa, Costa Rica's Rebeca Grynspan, Guyana's Carolyn Rodrigues-Birkett and Senegal's Macky Sall.
Even if one candidate gets the required nine votes on the 15-member Security Council -- which chooses the future UN chief before submitting the name for approval by the General Assembly -- that candidate must also avoid a veto.
The United States, Britain, China, France and Russia thus have the future of the world body in their hands.
Here is a look at the contenders:
- Michelle Bachelet -
A Chilean socialist brutally tortured by the regime of Augusto Pinochet, Bachelet became her country's first woman president in 2006.
She went on to be the UN rights chief, a sensitive role in which she alienated some countries, especially China, which mauled her for reporting on Beijing's alleged abuses of the country's Uyghur minority group.
Bachelet, 74, has said the UN secretary-general should have the "moral voice" to lead, even under pressure from major powers.
She is backed by Mexico and Brazil -- but Chile withdrew its support after far-right President Jose Antonio Kast took office. The United States has criticized her, especially on her support for abortion rights.
- Maria Fernanda Espinosa -
Espinosa, a former foreign minister of Ecuador, has called the United Nations the only universal platform for handling humanity's common challenges, but also wants deeper reform of the world body.
The 61-year-old, a previous president of the UN General Assembly, has suggested creating an "early warning" system to detect and flag signs of impending conflict.
Espinosa's candidacy has been backed by Antigua and Barbuda, but not her own country.
- Rafael Grossi -
The 65-year-old Grossi, an Argentine career diplomat, has led the International Atomic Energy Agency since 2019, propelling him into the middle of the battle over Iran's nuclear program as well as the Russian occupation of Ukraine's Zaporizhzhia nuclear plant.
He did not step down from his current post to campaign for secretary-general, saying it would be a "dereliction of duty."
Some observers say he's been more strident in pushing for UN reform, and believes the UN secretary-general should take a more hands-on approach.
- Rebeca Grynspan -
Grynspan -- Costa Rica's former vice president -- leads the UN trade and development body UNCTAD, but took leave from her duties to campaign for the top UN post.
In 2022, she pulled off a diplomatic feat by brokering the Black Sea Grain Initiative between Moscow and Kyiv to allow grain exports following Russia's invasion.
The 70-year-old plays up her personal story as the daughter of Jewish parents who she says "barely survived" the Holocaust before emigrating to Costa Rica.
She has lamented that the UN has become what she calls a "risk-conservative organization."
- Carolyn Rodrigues-Birkett -
Rodrigues-Birkett, the 52-year-old former foreign minister of Guyana, is currently her country's ambassador to the UN.
She has said the United Nations needs to restore its "credibility" on the global stage.
"The UN is judged through the peace and security lens. But the UN is much wider than that and it's delivering in many, many areas -- in development, in human rights," she said.
- Macky Sall -
Macky Sall, 64, is the only candidate not from Latin America, from where the next UN boss should come, according to convention.
The former Senegalese president has stressed the link between peace and development.
Proposed by Burundi, the current chair of the African Union, Sall finally got the backing of his own country this week.
Senegalese authorities accuse him of bloodily repressing violent political demonstrations that left dozens dead between 2021 and 2024.
