Laotian government officials and executives of the Thai company hired to build the dam will officially inaugurate the multibillion-dollar project at a ceremony Wednesday in northwestern Laos, The New York Times reported.
Electricity from the project will be sold to Thailand and will provide Laos with billions of dollars in revenue.
The Mekong River passes through parts of Thailand, Cambodia and Vietnam before emptying into the South China Sea.
The portion of the river where the dam will be constructed is entirely inside Laos but countries along the river signed an agreement in 1995 to consult one another before proceeding with large river projects.
The U.S. State Department said Monday that it was concerned those countries "have not reached consensus on whether the project should proceed."
Laos said it has satisfied the requirement to consult with other countries.
"This is not an international issue," an official of the Laotian Department of Electricity said. "It's more an internal affair."