The incident adds to a series of attacks that have raised fears of a growing Southeast Asian piracy menace.
The unnamed tanker was travelling from Singapore to Thailand and it was attacked near the Malaysian resort island Tioman in the South China Sea, the International Maritime Bureau's (IMB) Piracy Reporting Centre said.
The ship's crew were locked in the engine room as the pirates siphoned off the tanker's cargo of lubricant oil to another vessel.
The ship and its crew - all unharmed - were released earlier today.
A month ago, IMB had expressed its concern over a growing trend of small tankers hijacks that took place in the first half of 2014.
In Southeast Asia at least ten known cases of coastal tankers being hijacked for their cargoes have been reported since April, according to Noel Choong, head of the Kuala Lumpur piracy centre.
About one-third of global trade flows through the strategic channel, which runs between Indonesia, Malaysia, and Singapore.
