After a nearly two-week trial in Santa Monica, jurors found the 88-year-old Cosby liable for the sexual battery and assault of Donna Motsinger.
Cosby's lawyer, Jennifer Bonjean, said in an email that they are disappointed and fully intend to appeal the verdict.
The decision came nearly five years after Cosby was freed from prison in Pennsylvania when the state Supreme Court threw out a criminal conviction based on similar allegations.
Motsinger had been a server at a restaurant in Sausalito near San Francisco who said in her lawsuit, filed in 2023, that Cosby had invited her to his stand-up comedy show at a theatre in nearby San Carlos.
Both were in their 30s at the time. She said Cosby gave her wine and two pills that she believed were aspirin, and that she was going in and out of consciousness as two men put her in a limousine.
"She woke up in her house with all her clothes off, except her underwear on - no top, no bra, and no pants," the lawsuit said. "She knew she had been drugged and raped by Bill Cosby."
In court filings, Cosby's lawyers argued the allegations rested almost entirely on speculation and assumption, saying Motsinger "freely admits that she has no idea what happened."
Jurors awarded Motsinger $US17.5 million in past damages and $US1.75 million for future damages, including "mental suffering, loss of enjoyment of life, inconvenience, grief, anxiety, humiliation, and emotional distress." They deliberated for just over a day.
They can award even more in a punitive damages phase of the trial that starts later on Monday.
Cosby did not testify at the trial, whose witnesses included Andrea Constand, the Temple University sports administrator he was convicted of sexually assaulting in a Pennsylvania criminal court in 2018. The state's Supreme Court threw out the verdict and Cosby was freed from prison after serving nearly three years of a three- to 10-year sentence.
Motsinger first made her allegations anonymously in a 2005 lawsuit filed by Constand.
The Associated Press does not typically name people who say they have been sexually abused unless they come forward publicly and consent to be named, as Constand and Motsinger have.
In 2022, a jury in Santa Monica awarded $US500,000 to a woman who said Cosby sexually assaulted her at the Playboy Mansion when she was a teenager in 1975.
Motsinger's lawsuit echoed allegations of rape, sexual assault and sexual harassment made by at least 60 women against Cosby, all of which he has denied.
The former stand-up comedy and television superstar once widely known as "America's Dad" became the first celebrity tried and convicted in the #MeToo era before his conviction was permanently thrown out when appeals court found he gave incriminating testimony in a deposition only after believing he had immunity from prosecution.
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