Now 89% of the population of the world have access to improved water supplies, up from 76% in the base year of 1990.
UN Secretary General Ban Ki-Moon hailed the achievement of halving the number of people without access to improved drinking water.
He said it was thanks to people who had seen it not as a dream, but a vital step to improve health and well-being.
Improvement to clean water supplies has not been even: 40% of those still without access to improved drinking water live in sub-Saharan Africa.
Worldwide, almost 800 million people still drink dirty water. But in the past 20 years, two billion people have gained access to improved drinking water.
While this was the first significant Millennium Development Goal (MDG) to be reached, the charity Water Aid says that the other part of the target, for safe sanitation, is more off track than any other MDG.
The biggest challenge for this target is in India, where more than half of the population, 626 million people, do not have access to a toilet.
The other MDGs, including those on reducing poverty and improving access to education, are unlikely to be hit by 2015. The global economic downturn and greater pressure from increased population have pushed success even further out of reach.
The Millennium Development Goal for access to clean water has been reached, ahead of the target date of 2015.