The 193-member United Nations General Assembly appointed former Portuguese prime minister Antonio on Thursday as the ninth secretary-general of the world body for five years from January 1, 2017.
Guterres, 67, will replace Ban Ki-moon, 72, of South Korea. Ban will step down at the end of 2016 after serving two terms.
Guterres was prime minister of Portugal from 1995 to 2002 and served as United Nations High Commissioner for Refugees from 2005 to 2015.
The socialist politician, who also served as UN refugee chief for a decade, is expected to play a more prominent role as the world's diplomat-in-chief than Ban, the South Korean former foreign minister who will step down after two five-year terms.
Guterres won unanimous support from the UN Security Council during a vote last week that capped the most transparent campaign ever held at the United Nations for the top post.
The 67-year-old polyglot campaigned on a pledge to promote human rights and enact reforms within the UN system, seen as clunky and too slow to respond to unfolding disasters.
His appointment comes at a time of global anxiety over the ongoing war in Syria, the refugee crisis and raging conflicts in South Sudan and Yemen.
The Security Council is deadlocked over Syria after two draft resolutions were defeated in separate votes over the weekend, one of which was vetoed by Damascus ally Russia.