State television said army naval commandos detained the vessel in the Gulf of Oman; shipping data indicates it is managed by a Chinese company.
Iran said the tanker was a sanctioned vessel attempting to disrupt the country's oil exports and national interests.
Earlier reports said the ship, also known as Jin Li, had loaded 474,063 barrels of Iranian fuel oil and was later directed toward Iran's southern coast.
Why did Iran's navy seize a tanker from its own secret oil fleet?
Can Iran force global shipping to pay millions in tolls to pass through the Strait of Hormuz?
With US-Iran clashes escalating, how will this conflict impact global food and fuel prices?