The UN's 193-nation General Assembly appointed Antonio Guterres on Friday to head the world body for a second five-year term.
The secretary general said he would work for a “greener, safer and better future” in his next term, which begins on January 1, 2022.
Mr Guterres has been praised for navigating the UN through the difficulties of the Donald Trump presidency – when relations were often strained– while weathering deepening US-China tension and the Covid-19 pandemic.
But critics say he is too lenient on human rights abusers.
“The pandemic has revealed our shared vulnerability, our interconnectedness and the absolute need for collective action,” Mr Guterres said.
“We need to do everything we can to overcome current geostrategic divides and dysfunctional power relations.”
Mr Guterres took office in January 2017, having previously served as Portugal’s prime minister from 1995 to 2002, and head of the UN refugee agency from 2005 to 2015.
As the world’s top diplomat, he has pushed for climate action, coronavirus vaccines for all and action in the fight against poverty.
“Equity needs to start now. Vaccines need to be available for everyone, everywhere, and we must create the conditions for sustainable and inclusive recovery both in the developed and developing world,” he said.
Mr Guterres began his first term only weeks before Donald Trump became US president, leading with a go-it-alone attitude and a disdain for multilateralism.
The US is the top UN financial backer, covering 22 per cent of its budget and around a quarter for its blue helmet operations.
US President Joe Biden has worked to revive multilateralism and has reversed some funding cuts to UN agencies made in Trump era.
US Secretary of State Antony Blinken on Friday called Mr Guterres a "capable leader in a demanding role".
"The UN is an indispensable anchor of the multilateral system," said Mr Blinken.
"It provides a vital framework through which member states work together to meet such unprecedented challenges as the Covid-19 pandemic, climate change, human rights, development, migration, and humanitarian crises."
But under the watch of Mr Guterres, the conflicts and humanitarian crises in Yemen and Syria have dragged on without resolution, while new emergencies have erupted in Myanmar and Ethiopia's northern Tigray region.
Louis Charbonneau, UN director for the campaign group Human Rights Watch, said that in his second and final term Mr Guterres could start "calling out" human rights abuses by powerful countries.
"Guterres's legacy will depend on his willingness to speak out for all the oppressed, wherever they are," he said.
Seven people nominated themselves as challengers to Mr Guterres, including former Ecuadoran president Rosalia Arteaga, but none of them had the necessary backing of a UN member state. Mr Guterres was the candidate for his native Portugal.
Despite reforms, the selection process for UN chiefs remains opaque and is micromanaged by the US, China and other big powers.