No reports have emerged so far about casualties or damage.
It is unclear whether a tsunami warning will be triggered.
Mark Zastrow, who lives in Seoul, tweeted: “Woah, earthquake in Pohang and just felt it in Seoul, a few seconds after I got the alert on my phone.”
According to a US Geological Survey (USGS) chart the intensity, it would have been felt in North Korea.
Reports of buildings shaking, books falling from shelves and frames falling off walls have emerged from Seoul about 270 km away from Pohang, as well as the island of Jeju.
The region is home to nuclear power plants and industrial facilities, but there has been no word yet of any damage to them.
South Korean officials are trying to assess the damage according to local media.
North Korea and South Korea are located in the region known as the Ring of Fire – which extends around the Pacific Ocean and is a hotspot for earthquake activity.