Its cargo is valued at $100m. The fate of the 25 crew members is not known.
Richard Mitchelson, a former special forces soldier and security consultant based in Singapore, told Al Jazeera, that the pirates have refined their technique over time.
"They have a lot of larger vessels," he said.
"They use a mother craft so they can position themselves in open water, then they use smaller, faster speedboats and use caving ladders to gain access to the decks [of the vessels they target]. Once they are on, it is pretty easy. They are well armed.
"They have access to GPS [global positioning system] and Sat phones ... I'm sure they would have some form of intelligence as to which vessels are coming their way."
The Sirius Star, weighing 318,000 tonnes and flagged in Liberia, is operated by Dubai-based Vela International.
The crew members come from Croatia, Britain, the Philippines, Poland and Saudi Arabia.
Oil prices rise
The tanker can hold up to two million barrels of oil - more than one quarter of Saudi Arabia's daily exports.
It had been heading for the US via the Cape of Good Hope at the southern tip of Africa, instead of heading through the Gulf of Aden and the Suez Canal.
Last week, the European Union started a security operation off the coast of Somalia to combat growing piracy and protect ships carrying aid agency deliveries.
It is the EU's first-ever naval mission.
Pirates are well organised in the Horn of Africa, the area where Somalia's northeastern tip juts into the Indian Ocean.
So far this year there have been 63 reported incidents of piracy and 13 vessels are still being held with a total of 275 crew members.
This year alone $30m has been paid in ransoms.
Somalia has had no effective government since the 1991 overthrow of Mohamed Siad Barre, the former president, touched off a bloody power struggle that has defied numerous attempts to restore stability.