Arirang News reports Seoul government officials as saying that the ship, named Lighthouse Winmore, came into Yeosu Port on November 24, where it was detained and inspected by customs officials.
The ship is thought to have transferred of some 600 tons of oil to a North Korean vessel, the Sam Jong 2, in international waters a month earlier, on October 19.
All ship-to-ship transfer of any goods for North Korea has been banned by the UN Security Council since September.
According to the Equasis data base, the Lighthouse Winmore is a 2014-built, 16,500 dwt chemical/oil products tanker. It is owned by Win More Shipping Ltd and managed by Lighthouse Ship Management Ltd, all of which share the same Guangzhou, China, address.
The Lighthouse Winmore last visited South Korea on October 11 where it was chartered by a Taiwanese company called the Billions Bunker Group to take a shipment of Japanese refined oil products to Taiwan.
But instead, reports Arirang, it is said to have transferred the oil to the North Korean vessel, as well as three other ships.
Information was gathered with the help of the U.S., and Seoul said that a report will be given to the United Nations.
Arirang says that officials described this incident as a clear example of the illegal network North Korea has created to violate sanctions, but said that it also sends a message to those that do trade with the rogue regime that they can get caught.
This incident follows on from other reports of Chinese vessels trading oil with North Korean ships in the open seas.