The surge of water submerged homes and forced villagers to seek safety on this roof
The collapse at the hydroelectric dam in Attapeu province late on Monday sent flash floods through six villages, Lao News Agency said.
More than 6,600 people have been made homeless, it added.
Pictures showed villagers stranded on the roofs of submerged house and boats carrying people to safety.
"The disaster has claimed several human lives [and] left hundreds of people missing," the agency reported.
Construction of the Xe-Pian Xe-Namnoy dam began in 2013 and it was due to begin commercial operations next year.
The main Thai stakeholder in the project, Ratchaburi Electricity Generating Holding, said it had received a report from the dam's operator that a "saddle dam", measuring 770m long and 16m high, had collapsed.
It said continuous rainstorms had led "a high volume of water" to flow into the reservoir.
"Currently, Xe-Pian Xe-Namnoy Power Company Limited and related agencies have evacuated the people who reside around the area," the statement added.
A spokesman for SK Engineering & Construction, a South Korean company with a stake in the project, told the BBC a small supply dam - not the main dam - had been partly lost due to "unexpected heavy rainfall".
"We still don't know the exact cause, but we believe parts of the upper part of the dam were lost... and water overflowed from the supply dam," he said.
Environmental groups have previously expressed fears over Laos's hydroelectric programme and its potential impact on communities and ecosystems downstream of dams.
Prime Minister Thongloun Sisoulith has postponed government meetings and gone to the affected area in Sanamxay district with senior officials to monitor relief efforts, state media said.
Local authorities have appealed to government bodies and other communities to help provide emergency aid for the victims such as clothing, food, drinking water and medicine.