The incident took place Thursday near the Malaysian resort island Tioman in the South China Sea as the tanker was travelling from Singapore to Thailand, the International Maritime Bureau's 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, it said. The ship and its crew - all unharmed - were released early Friday.
Under a routine practice, the Kuala Lumpur-based piracy centre declined to release the name of the ship or its owners.
The attack was the tenth in the South China Sea since April, said Noel Choong, who heads the centre. He called that number "abnormal."
"We urge regional countries to cooperate to investigate and stop this menace," he told AFP. "We need to stop it before it starts spreading."
Piracy was a problem in Southeast Sea for centuries, but stepped-up patrols by regional countries were credited with bringing a sharp decline in attacks in recent years.
But a spate of daring hijackings in recent months - usually targeting tanker cargoes - has fanned concern that the region's vital shipping lanes could once again become a hotspot for piracy, particularly the Malacca Strait.
About one-third of global trade flows through the strategic channel, which runs between Indonesia, Malaysia, and Singapore.