The aftershock occurred at 6:22 a.m. in waters about 38 kilometers off the island's southwestern city of Seogwipo, near the epicenter of Tuesday's quake, according to the Korea Meteorological Administration (KMA).
Officials said the quake was the 18th and the biggest aftershock of Tuesday's earthquake.
Local fire authorities said no damage was expected from the aftershock. (Yonhap)