The Stena Impero tanker has departed from its port in Iran, two months after it was seized.
Iran's marine and port authority said the Stena Impero was heading to international waters.
Hours earlier, the tanker had begun transmitting its location for the first time in weeks just outside the Iranian port of Bandar Abbas, where it had been held since its seizure.
The ship tracking site MarineTraffic.com showed the ship moving south away from Bandar Abbas at a speed of 12 knots - around 14mph.
On Monday, Iranian government spokesman Ali Rabiei had told journalists the Stena Impero could leave.
"Based on a friendly approach that allows forgiving mistakes, ground for freedom of the tanker has been paved and it can move," Mr Rabiei said.
The tanker has been kept in the Iranian port Bandar Abbas since July 19.
Iran seized the tanker in the Strait of Hormuz , the narrow mouth of the Persian Gulf through which 20 per cent of the world's oil passes.
The seizure came after authorities in Gibraltar seized an Iranian tanker carrying 130 million dollars (£104 million) in crude oil on suspicion of it breaking European Union sanctions on Syria.
Gibraltar later released the tanker, then called the Grace 1, after it said Iran had promised the ship would not go to Syria.
The ship seizures came after months of heightened tensions in the Persian Gulf, sparked by US president Donald Trump's decision over a year ago to withdraw from Iran's nuclear deal with world powers and impose crippling sanctions on its vital oil trade. Iran since has begun breaking terms of the deal.
