The revelation raises questions about more than 140 other tanks that hold tens of millions of gallons of radioactive waste, the Seattle Times reported Friday.
The announcement by the Energy Department upset Gov. Jay Inslee, who said state officials were told years ago the problem "was under control."
The single-shell tank in question was assumed to have been leaking for decades, so pumpable liquids were removed in 1995 in what the Energy Department called an "interim stabilization."
The tank still contains about 447,000 gallons of sludge.
Although $12 billion is being spent to construct a plant to treat high-level radioactive waste, Inslee said the federal government needed to find funds to plug the leaking tank, check the conditions of other tanks and build more double-shelled tanks.
The governor said the leak did not pose any immediate threat because it would take years to reach the groundwater.
A 2008 report by the U.S. General Accounting Office said the tanks had leaked about 1 million gallons of waste into the ground over the years.
Hanford managers intentionally released 121 million gallons of radioactive liquid waste directly into the ground between 1946 and 1966, the GAO said.