Bachelet, who was Chile's first woman president, ended a speech at the closing session of a two-week meeting of the Commission on the Status of Women on a personal note. However, she did not mention Chile's presidential race or give any specifics on when she was leaving or what her future plans are.
"This will be my last CSW," she said. "I'm going back to my country."
U.N. Secretary-General Ban Ki-moon said Bachelet had informed him of her intention to step down and expressed "tremendous gratitude for her outstanding service."
"Michelle Bachelet was the right person in the right job at the right time," he said. "Her visionary leadership gave UN Women the dynamic start it needed. Her fearlessness in advocating for women's rights raised the global profile of this key issue. Her drive and compassion enabled her to mobilize and make a difference for millions of people across the world."
Ban said her achievements include new steps to protect women and girls from violence, new advances on health, and a new understanding that women's empowerment must be at the core of what the United Nations does.
"This is a stellar legacy, and I am determined to build on it," he said.
Bachelet is widely expected by Chileans to be a candidate in the Nov. 17 presidential election, though she did not address the political angle. Recent polls have said that 54 percent of voters support her, and the center-left opposition views her as its best chance to defeat conservative President Sebastian Pinera and regain power.
"I'm happy for the country," Osvaldo Andrade, leader of the Socialist party told CNN Chile. "There must be a nervous breakdown in downtown Santiago. It's an unequivocal sign."
Carlos Larrain, head of the conservative National Renewal party, said a Bachelet candidacy "will be healthy for the system" by giving voters a choice between the social welfare policies of her previous term and those of Pinera's right-of-center government.
Chile is respected for its fast-growing economy and transparent institutions, but it also has the worst inequality rate among the 34 countries of the Organization for Economic Co-operation and Development and widespread protests of inequalities have harried the administrations of both Bachelet and her successor, Pinera.
Millions of Chileans have staged widespread and frequent protests demanding a wider distribution of Chile's copper riches, free education and the return of ancestral lands to Mapuche Indians in a southern region where members of Chile's largest indigenous group often clash with timber companies and landowners.
The 62-year-old Bachelet, who was elected president of Chile in 2006 and served one term that ended in 2010, is the daughter of an air force general who was tortured to death for opposing the coup that put Gen. Augusto Pinochet in power. She and her mother also were arrested and then exiled. After returning to Chile, Bachelet became a pediatrician and then entered politics, serving as minister of health and as minister of defense before winning the presidency.
After she left office, Secretary-General Ban Ki-moon chose her as the first head of UN Women, which combined four U.N. bodies dealing with the advancement of women under a single umbrella.
Bachelet was tipped as a possible leader of the agency immediately after the General Assembly voted unanimously in July 2010 to create UN Women. But U.N. officials said she initially told them she wasn't interested because she wanted to remain active in Chilean politics after stepping down from the presidency in March 2010 with high approval ratings.
The secretary-general made no mention when he announced her appointment of what changed Bachelet's mind. But Ban said: "I am confident that under her strong leadership, we can improve the lives of millions of women and girls throughout the world."
Bachelet made her announcement minutes after more than 130 countries adopted a 17-page U.N. blueprint to combat violence against women after two weeks of tough negotiations at the Commission on the Status of Women. She called the document "historic."