The General Assembly adopted a budget of $5.396 billion for 2018-2019, slightly below the $5.4 billion that UN Secretary-General Antonio Guterres had sought.
The United States is by far the largest contributor to the UN budget, providing for 22 percent of the core budget.
US Ambassador Nikki Haley said in a statement that "inefficiency and overspending" at the world body were "well known."
Haley said the budget negotiations had generated several "successes" with financial cutbacks and a reduction of the UN's "bloated management and support functions."
"We will no longer let the generosity of the American people be taken advantage of or remain unchecked," she said.
"This historic reduction in spending -- in addition to many other moves toward a more efficient and accountable UN -- is a big step in the right direction."
"While we are pleased with the results of this year's budget negotiations, you can be sure we'll continue to look at ways to increase the UN's efficiency while protecting our interests," she added.
The UN's operating budget is separate from its peacekeeping budget, which was cut by $600 million this year under pressure from the administration of US President Donald Trump.
The push for deeper cuts comes as Guterres is trying to build support for his plans for reform of the UN bureaucracy.
Guterres' budget proposal was $200 million below the 2016-2017 biennial budget.
During a meeting held on the sidelines of the annual General Assembly debate in September, Trump said the United Nations had failed to reach its "full potential due to bureaucracy and mismanagement."
"We are not seeing the results in line with this investment," he said.
Last week, Trump threatened to cut funding to countries that supported a UN resolution rejecting his decision to recognize Jerusalem as Israel's capital.