S-Oil said the crude-oil supply agreement with Saudi Aramco would ensure its refineries are fully supplied with crude for the next 20 years.
"(A) 20-year, long-term crude supply contract is highly unusual in the market where one-year supply contract is the norm, which indicates that it would not have been possible had it not been for the mutual trust built between the two companies for a span of more than 20 years and the strong friendship between Korea and Saudi Arabia," the South Korean company said in a statement.
S-Oil said crude oil deliveries from Saudi Aramco will be "highly conducive" to the country's energy security.
The agreement follows a visit to the region by South Korean President Lee Myung-bak. Riyadh had said it was ready to help Seoul with any additional crude oil demands.
Iranian crude had made up about 10 percent of South Korea's imports, though the country said last month it was looking for alternatives as the economic pressure on Tehran increases. South Korea depends on imports to meet its entire demand for oil.