A 5.5 magnitude natural disaster struck the southeastern city of Pohang in North Gyeongsang Province on Wednesday, the state weather agency said.
The Korean peninsula rarely has to worry about significant quakes.
South Korean officials are assessing the situation.
Reports of shaking buildings and toppling furniture flooded in from Seoul to Jeju Island. No immediate damage was reported in the firm's steel mills.
Images posted on social media showed shattered storefronts, goods tumbled off store shelves, cars smashed by fallen bricks and cracks on the ground.
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.
Buildings in Pohang, North Gyeongsang Province, are seen after an quake of magnitude 5.5 hit the area.
Pohang is some 35km from the Wolseong Nuclear Power Plant, which has six operational reactors.