Sales in January at established McDonald's restaurants around the world fell 1.9%, which was more than analysts expected.
A rise of almost 1% in the US failed to compensate for sliding sales elsewhere.
In Europe, sales fell by 2.1%, as the eurozone's debt crisis continued to hit consumer spending in Germany and France especially.
Sales slumped by 9.5% in the region that encompasses Asia, the Middle East and Africa.
McDonald's cited a weakness in demand in Japan as one of the causes, as customers increasingly choose to eat at home.
In China the company has suffered from supply chain problems, and said 2013's later New Year had hit sales.
It is only the second time in nine years that McDonald's has reported a fall in monthly sales - the last being in October 2012.
The figures come amid intensifying competition in the fast food sector from rivals like Subway and Burger King.
They have taken on rival McDonald's - which dominated the sector for decades - by rolling out their own versions of products like premium milkshakes and wraps.
The company's president and chief executive Don Thompson said McDonald's is focussed on meeting the needs of all of its customers.
"While January's results reflect today's challenging environment and difficult prior year comparisons, I am confident that our unwavering commitment to delivering an exceptional restaurant experience will enhance our brand's relevance and drive long-term results," he said.
Last month the company warned it was expecting a negative sales figure for January, and said growth would remain under pressure in the near future.
McDonald's sales figures take into account restaurants open at least 13 months, removing any volatility as a result of newly opened and closed locations.
More From Sky News