Armed men wearing masks reportedly boarded the vessel St Nikolas, close to the Omani port of Sohar, and ordered it to sail to an Iranian port.
Iranian state media, quoting the navy, said the seizure was in retaliation for the ship and oil it had aboard being confiscated by the US last year.
The St Nikolas was in transit between the Iraqi port of Basra and its intended destination in Turkey.
UK Maritime Trade Operations said on Thursday it had received a report that four to five “unauthorised persons”, reportedly wearing “military style black uniforms with black masks”, had boarded the ship at 03:30 GMT.
They added that communications with the vessel had been lost and the authorities were investigating.
Iranian state media quoted the army as saying the vessel is American, although it is Greek-owned.
The company that manages it, Empire Navigation, said it was loaded with 145,000 tonnes of crude oil and was carrying 18 Filipinos and one Greek citizen as crew.
The St Nikolas was seized in April by the US under its previous name, the Suez Rajan, as part of sanctions enforcement against Iran.
Suez Rajan Limited, which formerly chartered the ship, later pleaded guilty to conspiring to violate sanctions by covertly selling and transporting oil abroad on behalf of Iran.
This latest act appears to be separate from attacks carried out by Houthi rebels from Yemen in the Red Sea on the opposite side of the Arabian peninsula.
Thursday’s incident was in a different ocean from the Houthis’ usual area of operations – where they have been attacking any ships they suspect of being linked to Israel in retaliation for the country’s actions in Gaza.
The US and UK have hinted they could take military action against Yemen’s Houthi rebels following the largest attack yet.
Carrier-based jets and warships shot down 21 drones and missiles launched by the Iran-backed group on Tuesday night.
The UN Security Council passed a resolution on Wednesday demanding an immediate end to the Houthi attacks.