A gas turbine plant includes a gas turbine, an exhaust heat recovery boiler, and a water supply system that is configured to supply water to the exhaust heat recovery boiler. The exhaust heat recovery boiler has an evaporator configured to use an exhaust gas to heat water, thereby generating steam, and a reheater that is configured to heat steam with the exhaust gas introduced from the evaporator. The water supply system has a water supply line that is configured to supply water from a water supply source to the exhaust heat recovery boiler, and a supplied water temperature regulator that is configured to regulate a temperature of supplied water, which is the water in the water supply line.