The 193 UN member countries agreed on Saturday to cut the global body's budget for only the second time in the past 50 years following a long night of negotiations.
An accord struck at dawn on Saturday saw the 2012-13 budget set at $5.15 billion, down from $5.41 billion in 2010-11.
The United States and crisis-stricken European countries had fought for cuts while developing countries had demanded spending be maintained.
"All budget years are tough. But this year was especially difficult," UN Secretary General Ban Ki-moon said, welcoming the accord and vowing more cuts in the coming months. "Governments and people everywhere are struggling."
US negotiator Joseph Torsella called the budget a "historic agreement," though he acknowledged it had taken "difficult negotiations." Nearly every day in the past week talks have finished at about 5:00 am.
This accord "is the first time since 1998 -- and only the second time in the last 50 years -- that the UN regular budget has declined in comparison to the previous budget’s actual expenses," said Torsella.
He called it a "budget for a strengthened, more efficient, and more effective United Nations that saves the American taxpayers millions of dollars and sets the United Nations on the path of real fiscal discipline and continued reform."
The United States has taken a tough line on UN spending, with Torsella this year railing at increased salary allowances for UN staff.
Ban acknowledged that the global body has to "cut fat." It has already cut posts and contracted out services in many departments at the New York headquarters.
"One year from now, I will return to you with greater cost savings," the UN chief vowed in a speech to the delegates who struck the budget accord.
Several of the UN's foreign political missions saw their budgets cut as part of the deal.
The UN peacekeeping department has a separate budget of about $8 billion a year. Funding for the Ivory Coast mission, where UN peacekeepers fought followers of incumbent Laurent Gbagbo after he refused to stand down, was cut in the General Assembly negotiations.