The Reventazon dam is expected to produce 305.5 megawatts of electricity, enough to power about 525,000 homes.
The government electricity board says the project is 99 percent completed, with two of the dam's turbine generators now connected to the national power grid. Tests are being completed to bring the third unit on line.
The country has now reduced its reliance on fossil fuel-burning plants by 90 percent, and says it hopes to use such plants only as backups.
Hydroelectric plants how supply about 75 percent of the country's power, geothermal plants about 13 percent and wind power about 10 percent.
Costa Rica has been shutting down some fossil-fuel generators. But it remains to be seen how drought or climate change could affect the energy output from the big dam projects.
The dams have also been criticized for their effects on the ecosystems of the nation's rivers.