A jury in Santa Monica, Calif., on Tuesday sentenced the 84-year-old former entertainer to pay plaintiff Judy Huth $500,000 in damages.
The 64-year-old accused Cosby of sexually assaulting her in 1975 when she was 16.
The attack took place in the famous Playboy Mansion, the villa of "Playboy" founder Hugh Hefner in Los Angeles.
According to Huth's attorney, Cosby first got the teen drunk and then took her to the Playboy Mansion.
In a bedroom he "threw himself" on her and tried "to put his hands in her pants".
When she told him she was on her period, Cosby forced her to masturbate him, the attorney said at the trial.
Defendant protests his innocence
Cosby denies any assault.
His attorneys challenged Huth's account of events during the civil trial, in part because the plaintiff changed the date of the assault from 1974 to 1975.
Huth filed her lawsuit in 2014.
The process was then put on hold because of a lawsuit against Cosby in the state of Pennsylvania.
Previously three years in prison for sexual abuse
In this process, the actor, who became world famous with the sitcom "The Bill Cosby Show", was found guilty in 2018 of sexually abusing a woman in 2004.
The court saw it as proven that Cosby drugged his victim in his home in the east coast metropolis of Philadelphia and molested the woman.
Cosby was sentenced to a minimum of three and a maximum of 10 years in prison.
However, he was released from custody in June 2021 after the Pennsylvania Supreme Court overturned the verdict on formal grounds.
Dozens more allegations of abuse
More than 60 women have accused Cosby of sexual abuse, but most cases are statute-barred.
The actor was revered in the US as "America's Dad" for decades.
As the amiable doctor and good-natured father of a family on The Bill Cosby Show, he was one of the country's most popular TV stars before becoming an outlaw over abuse allegations.