Its crew - mostly from Myanmar and China - were being questioned by the customs office and the intelligence agency.
South Korea is holding a second ship on suspicion of supplying oil to North Korea in violation of global sanctions meant to punish Kim Jong-un's regime for its nuclear weapons and missile programmes.
The vessel, named KOTI, was seized at Pyeongtaek-Dangjin port, on the west coast south of Incheon, an official told Reuters on condition of anonymity due to the sensitivity of the information.
While UN measures limit the amount of refined and crude oil member states can send to the DPRK, the September resolution also bans member states from transferring cargos at sea.
The seizure follows that of the Lighthouse Winmore, another oil tanker which Seoul claims supplied fuel products to North Korea in October, in breach of UN Resolution 2375. We are also working closely with relevant countries and ministries for a thorough implementation of the United Nations resolutions.More news: Big Ten Bowl Preview: 2017 Playstation Fiesta Bowl
The Lighthouse Winmore is one of 10 ships the USA has asked the Security Council to blacklist for violating sanctions against North Korea. One was a North Korean vessel, but the Treasury didn't identify where the other ship was from. It is unclear when exactly KOTI was seized, but customs officials decided on December 21 to deny the ship permission to leave the port, a local maritime official told South Korean news agency Yonhap.
The Lighthouse Winmore was one of 10 ships the United States asked the United Nations to ban from worldwide ports this month over its alleged dealings with North Korea, according to Reuters.
Earlier this month, the US requested the U.N. Security Council to deny access to ports worldwide to 10 ships suspected of violating trade sanctions on North Korea.
Last week the UN Security Council unanimously adopted new sanctions on North Korea in response to Pyongyang's November 29 ballistic missile test, seeking to further strangle its energy supplies and tighten restrictions on smuggling and the use of North Korean workers overseas.
Additional reporting from Newsy affiliate CNN.