The ex-Soviet country's presidential press service said a state commission approved on June 27 the Rogun project seen as vital to the water-rich republic's efforts to step up electricity production.
The dam on the Vakhsh river, which at a planned 335 metres would be the tallest dam in the world, was begun during Soviet times but faces major opposition from downstream Uzbekistan.
Tajikistan's more populous and militarily powerful neighbour depends on water that flows through Tajikistan for its agricultural sector.
Tajikistan views Rogun as a means of ending regular winter power outages in the republic and increasing electricity exports to the wider region.
The World Bank has endorsed the project after carrying out extensive economic, environmental and technical studies but has not offered to finance construction.
Tajikistan is the poorest of the five Central Asian countries that gained independence from the Soviet Union in 1991, with up to half of working-age men leaving the country to find employment in Russia and other countries.