The first of the two units built by Chinas Sinohydro was successfully synchronised into the national grid late Monday, the Zimbabwe Power Company (ZPC) said.
"Power will be progressively fed into the grid until it reaches 300 MW," ZPC said in a statement.
The ZPC has said it expects the second 300 MW thermal unit to start generating power in October, bringing new generation capacity to 600 MW.
The southern African country is expanding its 920 MW Hwange thermal power station by adding two 300 MW units at a cost of $1.4 billion, with 85% of the funding coming from China.
Zimbabwe is currently generating less than half of its 1,700 MW demand as the old thermal units at Hwange, commissioned between 1983 and 1987, frequently break down and are performing below capacity.
Low water levels due to inadequate rains have seen generation from the country's other major plant, the 1,050MW Kariba South hydro station, being capped at a third of its capacity.
In December, the government announced incentives to help accelerate 1,000MW solar projects worth $1 billion planned by independent power producers, seeking to ramp up renewable power generation ami..d a funding freeze on coal-fired power projects as the world shifts away from the polluting fossil fuel.