Witnessed by senior diplomats of member states and senior UN officials, Guterres repeated the oath after current General Assembly President Peter Thomson.
With his left hand putting on a copy of UN Charter and his right hand up in the air, Guterres pledged as follows.
"I, Antonio Guterres, solemnly swear to exercise in all loyalty, discretion and conscience the functions entrusted to me as Secretary-General of the United Nations, to discharge these functions and regulate my conduct with the interests of the United Nations only in view, and not to seek or accept instructions in regard to the performance of my duties from any government or other authority external to the organization."
On Oct. 13, the UN General Assembly appointed Guterres as the 9th secretary-general of the United Nations to succeed retiring Ban Ki-moon who is to step down at the end of this year.
The five-year term for Guterres will start on Jan. 1, 2017 and end on Dec.31, 2021.
Guterres, a 67-year-old politician, served as UN High Commissioner for Refugees from June 2005 to December 2015. He was the prime minister of Portugal from 1995 to 2002.
After swearing-in, Guterres told the 193-member UN General Assembly that he is deeply honored by the trust and confidence member states have placed in him, and is "determined to be guided by the purposes and principles of the Charter."
Guterres said the priority for the United Nations is to address root causes of multiple global challenges through three pillars -- peace and security, sustainable development and human rights.
He pledged that he will lead the UN to reform its system to better support the worlds sustainable development agenda as well as the Paris climate deal.
He also promised to conduct a management reform to make the United Nations "nimble, efficient and effective, adding that the organization" must focus more on delivery and less on process; more on people and less on bureaucracy."
Speaking of Chinas role in UN affairs, Guterres told reporters here that "China is strongly engaged in creating or helping creating conditions for an effective multi-lateral approach to global development."
Noting his recent visit to China, Guterres said it was very clear that China has committed a lot to strengthening multilateralism around the world by creating a new bank and relative funds as well as the Belt and Road Initiative.
Guterres swearing in came after the UN General Assembly paid tribute to Ban for his decade-long service for the largest international organization.
Ban said he has been honored to serve alongside many courageous and talented women and men of the United Nations over the past ten years and he has seen "the power of international cooperation in taking on the most pressing challenges."
Ban wished the new secretary-general and all the UN member states "peace, prosperity and every success and concluded his speech with saying "thank you" in six UN official languages.
Ban served two five-year terms since 2007.(*)