The quake, which was felt across much of the country including in the capital Seoul, struck at the shallow depth of nine kilometres (six miles) near the southeastern industrial city of Pohang at around 2:30 pm (0530 GMT), the Korea Meteorological Administration said.
The Korean peninsula rarely has to worry about significant quakes.
But seismic activity is closely monitored because a spike in activity is often the first indication that North Korea has carried out a nuclear test.
The South Korean port city of Pohang is home to the headquarters of Posco -- the country's top, and the world's fourth largest, steelmaker. No immediate damage was reported in the firm's steel mills.
Photos and video footage sent to local TV stations showed crumbled street walls and furniture violently shaking inside people's homes and people rushing out of buildings in panic.
There were no immediate reports of injuries.
Emergency centres nationwide were flooded with thousands of calls seeking information, while Kakao Talk -- the South's top mobile messenger application, used by 42 million people -- reported disruption in services due to heavy traffic.
South Korean President Moon Jae-In, on his way to Seoul after attending the Asean summit in Manila, is set to hold an emergency meeting upon arrival, his office said.
Local nuclear reactors were operating without disruption, Yonhap news agency said, citing officials at Korea Hydro and Nuclear Power.
The most powerful quake recorded in the south was a 5.8-magnitude tremor that hit the southeastern city of Gyeongju in September last year.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.