No casualties have been reported and it is still unclear whether a tsunami warning will be issued.
An quake of 5.5 magnitude on the Richter scale has hit the southeastern city of Pohang, the Korean Times reported.
Nuclear reactors in the region operated normally when the nation's strongest-ever natural disaster at magnitude 5.8 hit Gyeongju in September 2016.
South Korea's top steel maker POSCO, based in Pohang, said it experienced no damage from the natural disaster so far. As of 3:00 p.m. local time, four minor injuries were reported, according to the firefighting agency.
Before the main tremor, earthquakes of magnitude- 2.2 and 2.6 struck areas near the Pohang city. The quake was felt across the nation, including Seoul about 370 km away from the port city.
According to TV footage, some buildings were externally damaged and windows were broken.
The weather agency said the seismic center for the initial quake is 36-point-one degrees north latitude and 129-point-37 degrees east longitude. Students were evacuated from school buildings and people stood on the streets after dashing out of office buildings and apartments.
There are nuclear power plants and industrial facilities in the area.