"We will no longer let the generosity of the American people be taken advantage of or remain unchecked," US ambassador the UN Nikki Haley said in a statement.
Haley added that "inefficiency and overspending" at the world body were "well known".
"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."
Haley said the budget negotiations had generated several "successes" with financial cutbacks and a reduction of the UN's "bloated management and support functions."
The negotiations had taken place on Sunday, with the UN General Assembly adopting a budget of $5.396 billion for 2018-2019, slightly below the $5.4 billion that UN Secretary-General Antonio Guterres had sought.
The budget cut works out to 5 per cent of the UN's operating budget.
"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," Haley added.
The United States is by far the largest contributor to the UN budget, providing for 22 per cent of the core budget.
The UN's operating budget is separate from its peacekeeping budget, which was cut by $600 million this year under pressure from the Donald Trump administration.