Cambodia's bomb squad was called in Thursday to remove a 2,000-pound US-made explosive uncovered in a riverbed opposite the Royal Palace in the capital Phnom Penh, a demining official said.
The AN-M66 bomb -- containing more than 500 kilograms of explosives -- was found during dredging and clean-up work at the meeting point of the Mekong and Tonle Sap rivers, the Cambodian Mine Action Centre said.
"If this bomb had exploded it could have caused huge damage to hotels, houses or even the Royal Palace. It's very lucky for Cambodia," director general Heng Ratana said on Facebook.
The bomb has now been removed from the capital to be defused, he added.
The United States dropped millions of bombs on Cambodia and Laos during the Vietnam War in the 1960s and 1970s in an attempt to hit communist bases.
But Washington went on to become a major aid donor as Cambodia emerged from the ashes of the Khmer Rouge genocide that saw two million people die from overwork, starvation and mass executions between 1975 and 1979.
The effects of the bombing campaign have long been felt, with around 20,000 Cambodians killed over the last four decades after stepping on landmines or unexploded ordnance. Twice as many have been injured, many losing legs.
The kingdom has vowed to clear all mines and unexploded ordnance by 2025.