The 44-year-old also admitted convincing Klatten to give him euro7 million ($9 million) by saying it was for the treatment of a girl left paraplegic after he hit her with his car.
The Munich state court found Sgarbi guilty of fraud and attempted blackmail of Klatten, who turned him in to police.
He was also found guilty of fraud and attempted blackmail for taking euro2.4 million ($3 million) from three other women -- identified by the initials H., R., and S. -- who were located by authorities in their investigation of the Klatten case.
Prosecutor Thomas Steinkraus-Koch praised Klatten for her bravery in stepping forward.
"The only victim who came to us and gave a witness statement is Mrs. Klatten. We had to find the others, with great difficulty," Steinkraus-Koch said.
Prosecutors said the women were the victims in a series of scams by Sgarbi, who trained as a lawyer, spoke six languages and worked as a banker at Credit Suisse until the mid-1990s. He also served as a reserve officer in the Swiss army, according to prosecutors.
According to Swiss court documents, a countess 50 years older than Sgarbi accused him of swindling her in 2001 but then had charges dropped after he returned 20 million Swiss francs ($17 million) to her, Germany's Stern magazine said.
Steinkraus-Koch confirmed that one case had been dropped in Switzerland and Sgarbi had been sentenced to six months probation on another case in the country.
"It was not the first time he was involved in such things in this manner," Steinkraus-Koch said.
Klatten did not attend the four-hour trial and her attorney made no statement to the court. Her spokesman was not immediately available for comment.
Her husband, Jan Klatten, has made no public comment on the case.
Sgarbi told the court he had specifically targeted Klatten, laying the groundwork for his scheme when he first met her at a spa near Innsbruck, Austria, in July 2007.
Using what prosecutors called a well-practiced scam, he showed up a month later at her vacation home in the south of France and their affair began. Then he told Klatten that he was involved in a car accident in the United States that left the girl paralyzed, and convinced Klatten to give him the cash for her treatment.
According to her police statement, Klatten was initially taken aback by the story, but eventually met him in a parking garage to hand over the euro7 million ($9 million) in cash, packed in a large cardboard box.
Sgarbi then demanded Klatten leave her family and invest euro290 million ($367 million) in a trust for him, according to the indictment. When she refused, he threatened to release secretly filmed videos of their affair.
Sgarbi demanded euro49 million ($62 million) not to tell her family, the heads of her companies and the media. He lowered the demand to euro14 million ($17.72 million) but Klatten instead went to the police in January 2007. Sgarbi was arrested shortly in Austria's Tyrolean Alps and has been held in detention since.
Another man arrested with him at the time, identified only as an Italian named Ernano B., is also wanted by German authorities as an accomplice in the case. He was sent to Italy where he was being sought on charges related to organized crime, however, and the request for his extradition has been refused.
Klatten is the daughter of the late BMW magnate Herbert Quandt and holds a 46 percent stake in the company along with her mother and brother. She also owns a 88.3 percent share of chemical company Altana.
Forbes magazine listed her as the 68th richest person in the world last year, with a personal fortune of around $9.6 billion (euro7.58 billion). The Quandt family is regarded in Germany as quasi-royalty.
"I deeply regret what has happened and apologize to the aggrieved ladies in this public hearing," Sgarbi told the court. Sgarbi's lawyer, Egon Gries, said his client would not answer any questions about where the money had gone or whether anyone had helped him.
Sgarbi had faced a possible 10-year sentence, but presiding Judge Gilbert Wolf said he decided on six years because Sgarbi's decision to confess had spared his victims having to testify in court.
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