A ceremony marking the start of construction of the $3.5 billion project would be held Wednesday, the government said, despite opposition from countries downstream that are concerned it will affect fish stocks and the livelihoods of millions.
Laos, landlocked and poor, is committed to a strategy of generating electricity from its rivers and selling the power to its neighbors.
The dam at Xayaburi is being built and financed by a Thai company, and nearly all of the electricity it is expected to produce has been pre-sold to Thailand, the BBC reported.
"I am very confident that we will not have any adverse impacts on the Mekong river," Laos Deputy Energy Minister Viraphonh Virawong said. "But any development will have changes. We have to balance between the benefits and the costs."
Two of the dam's biggest opponents are Cambodia and Vietnam, who have voiced worries about its impact on fish migration and the flow of sediment downstream.
Viraphonh said studies and modifications to the original dam design had addressed those concerns, so that sediment will be allowed out of the bottom of the dam periodically through a system of gates and elevators and ladders will help fish travel upstream.
Environmentalists said they weren't convinced.
Kirk Herbertson of the environmental campaign group International Rivers said experts believe it is doubtful fish passages could work on the Mekong and "on the sediments issue, Laos is also jumping to conclusions."
"Laos is playing roulette with the Mekong, and trying to pass its studies off as legitimate science," he said.