The 66-foot twin-masted vessel had no name and no other identification markings and was discovered by coastguards on a routine patrol.
Onboard the yacht there was a half-eaten meal as well as maps of the Mediterranean, piles of clothes and a punctured tender.
The schooner - which was described by coastguards as of a "classic" design - was found drifting off the coast of Punta Volpe on the island of Sardinia, known as a millionaire's playground.
Strong currents were pushing it towards rocks and coastguards boarded the vessel just in time and managed to attach a line and tow it to the port of Olbia where it was being examined.
A coastguard spokesman said that they and police were trying to establish whether the yacht had been intentionally abandoned and what had happened to the crew.
Officials had discovered a plaque with the name "Bel Amica" but said that initial checks with shipping registers had found no yacht with that name.
No vessels have been reported missing or stolen, which has added to the mystery.
Yesterday, a spokesman at the harbour-master's office in Olbia said: "At the moment the discovery is a complete mystery and we are investigating with police.
"There was no name and no registration markings or documents were found, but a proper search is now being carried out with the yacht in harbour.
"She is in perfect condition and is a classic twin-masted schooner worth at least 450,000, if not more.
"She was found drifting off Punta Volpe and there was no sign of the crew at all.
"Onboard there was a half-eaten meal, maps of the Mediterranean and piles of clothes.
"It gave the impression of being abandoned very quickly, but for what reason we just don't know.
"At this stage we cannot rule anything out - it could be piracy, it could be the crew had a problem and abandoned the yacht, we just don't know.
"A plaque with the name Bel Amica was found but we can find no trace of a similar yacht with that name.
"It's certainly the sort of yacht you would get round here in the summer as the owner must be very wealthy and we get a lot of wealthy visitors in the summer."
The Mediterranean island is a popular destination for VIPs. Visitors this summer have included Prince Andrew - who spent an evening at an exclusive nightclub.
Russian Oligarch and millionaire Chelsea owner Roman Abramovich has also been seen there this summer on board his yacht Pelarus.
Also, designer Roberto Cavalli entertained David and Victoria Beckham on his yacht there.
Yesterday the mystery yacht was attracting dozens of onlookers at Olbia. Police had to put up barriers to keep back crowds of locals and tourists who had gathered to take a look.
Century-old puzzle still to be solved
THE mystery of the Mary Celeste remains unsolved to this day, but why she is remembered by the wrong name has a simple explanation - a story by Sir Arthur Conan Doyle.
The vessel was actually called the Mary Celeste and her tale was popularised by the Scots author.
In 1884, he published a story which drew on the original incident and called the ship the Marie Cleste.
The real Mary Celeste was found drifting off the coast of Portugal on 4 December, 1872. She had set sail from New York for the Italian port of Genoa, carrying a cargo of industrial alcohol, but was found drifting in the Atlantic with no sign of her crew.
Bloodstains were found along three railings, and scratches along one. A sword was discovered hidden under the captain's bed, smeared in blood. Some reports later said that the crew's tea was still warm when the ship was discovered - fictional details from Doyle's story.
In reality, the last entry in the ship's log was written eleven days before the discovery of the empty 280-ton brigantine.