The Sangtuda 2 power plant, south of the Tajik capital Dushanbe, should ultimately generate one billion kilowatt-hours of power a year, Tajik President Imomali Rakhmonov said.
Rakhmonov said the plant will provide electricity to Afghanistan, Iran, and Pakistan, as well as Tajikistan.
The Tajik president was joined at the opening ceremony by Iranian Energy Minister Parviz Fattah and Afghan Energy Minister Ismail Khan.
Fatah said the project is the biggest example of cooperation between Iran and Tajikistan.
The new plant is part of a larger, 670-megawatt hydroelectric complex that has been in various stages of construction since the late 1980s.
(compiled with agency reports)