The Guards said the vessel was carrying 7,200 tonnes of "oil chemical products" and that the detained crew were from South Korea, Indonesia, Vietnam and Myanmar.
On Tuesday, the MT Hankuk Chemi was seized by Iranian forces near the strategic Strait of Hormuz, detaining its crew of 20 people, with Iran stating that the ship had violated environmental rules.
This incident comes amid rising tensions over Iranian funds frozen in South Korean banks, due to U.S-imposed sanctions.