The country’s navy said the explosive device remained on the 3,300–dwt Lapin (built 1978) following the raid on Friday, in which 2,000 tonnes of oil were stolen.
The Bangkok Post reported that all 15 crew members were unharmed.
The navy has sent bomb disposal experts to the ship, which was anchored 11km from Pak Bara.
Lapin's captain Theekhathat Charoensuk sought help from another vessel after navigating the tanker into Thai waters on Sunday.
He said his ship was robbed in the Strait of Malacca on Friday evening.
Up to eight pirates, believed to be Indonesian, stopped the vessel and forced the crew to surrender.
Three were armed with guns, while the others carried swords.
The pirates then manoeuvred a larger vessel to the side of the tanker, before siphoning off the fuel.
They then planted a homemade bomb on the bridge.
The captain said the device looked like TNT held together by electrical wire.
The vessel is operated by Smooth Sea of Thailand.
