The tanker had been hijacked by eight of its own crew who wanted to sell its cargo to locals in the region.
The hijackers were apprehended by Thai authorities on November 27 and the vessel anchored 400 metres off the coast of Narathiwat, 700 kilometres south of Bangkok.
On Friday, strong winds and heavy seas broke it free, causing it to run aground on a local beach, officials at Narathiwat's central police station said.