The International Confederation of Free Trade Unions (ICFTU) said the alarming statistic is true despite the fact that more women have entered the workforce than men in recent years.
"The global figures show 2.8 billion people working in 2003 and of that 39% - 1.1 billion - were women, the highest proportion ever," said the ICFTU on Tuesday.
The group also said that women face "growing obstacles" in the workplace as the best paying jobs are overwhelmingly reserved for men.
Differences in salaries between the genders "keep women in poverty", according to the ICFTU, which is partly explained by the fact that women often do not have full-time positions.
But even in cases of similar work women earn less than men , both in the developing world and in industrialised nations.
In the United States, female nurses earn 10% less for the same job as their male counterparts. And in France, there is a 25% discrepancy between the wages of men and women.