The former prime minister of Portugal was the UN high commissioner for refugees from 2005 to 2015. In his inaugural speech, he reaffirmed the importance of conflict resolution and called for an end to the war in Syria.
The United Nations General Assembly on Monday swore in Antonio Guterres as the ninth UN Secretary-General.
"From the acute crises in Syria, Yemen, South Sudan and elsewhere, to long-running disputes including the Israeli/Palestinian conflict, we need mediation, arbitration, as well as creative diplomacy," Guterres told the 193-member body.
Guterres, 67, will replace Ban Ki-moon, 72, of South Korea on January 1, 2017.
Guterres is a former prime minister of Portugal and spent 10 years as the UN High Commissioner for Refugees.
He is seen by many as a skilled politician, whose talents will be tested on major issues like Syria.
"As part of my good offices I am ready to engage personally in conflict resolution where it brings added value," he told the General Assembly.
He has also said he is committed to reforming the United Nations – a move that might appeal to one of the organisation's most vocal critics – incoming US President Donald Trump.
But it's Trump's controversial policy proposals that will likely present a greater challenge for Guterres.
These proposals include Trump's desire to ban Muslims from entering the US, sceptical views on climate change, and his support of harsh interrogation techniques like waterboarding.
The new secretary-general's first day on the job will be January 1, 2017, and he has told the UN General Assembly that the UN must be ready for change and that change will focus on people, not bureaucracy.