Imports from January to May dropped 15.7 percent from the same period last year, according to the preliminary figures from the state Korea National Oil Corporation.
May’s imports from Iran were down 47.3 percent month-on-month.
South Korea has in the past imported about 10 percent of its crude needs from Iran.
It is cutting purchases in return for a waiver from US sanctions which aim to pressure Tehran to abandon a suspected nuclear weapons programme.
Earlier this month, two South Korean shipping companies said they had suspended importing Iranian crude due to a separate EU embargo set to come into force from July.
SK Shipping, which handles the needs of refiner SK Energy, said it shipped its last cargo from Iran early this month and it would arrive by the end of June.
Hyundai Merchant Marine said it had not carried any Iranian crude for Hyundai Oilbank in June.
SK Energy and Hyundai Oilbank are the only Korean refiners still importing Iranian oil. They said the suspension was temporary as Seoul seeks an exemption from the EU measures.
Under sanctions which come into effect on July 1, the EU will ban European firms from insuring or reinsuring tankers transporting Iranian oil anywhere in the world.
This could mean Korean refiners, who rely on European companies for insurance, are unable to import such supplies.
— Dow Jones Newswires contributed to this report —
