Reactor No. 6 at the Younggwang Nuclear Power Plant shut down automatically after a warning system sounded Monday afternoon local time.
An official at the state-run Korea Hydro and Nuclear Power Co. told the Yonhap news agency an investigation started to "confirm the exact cause of the automatic shutdown of the reactor."
The company added there was no indication radiation was leaking from the 1,000-megawatt nuclear reactor, located about 200 miles south of Seoul. There are six reactors at Younggwang. Reactor No. 6 came online in 2002.
A reactor at the country's Gori NPP was shut down for safety inspections in March and hasn't resumed operations, Yonhap adds.
Nuclear power has come under scrutiny since the March 2011 disaster at the Fukushima Daiichi nuclear power plant in Japan. Fukushima melted down after a magnitude-9 earthquake and resulting tsunami.
South Korea gets 40 percent of its electricity from the 21 reactors in service.