The verdict is a major defeat for the 84-year-old Cosby, writes the news agency AP.
Barely a year ago, he was released from prison after serving two years of a sentence for sexual exploitation, this after a court ruled that the prison sentence was invalid.
A large number of accusations from other women have previously ended in financial settlements.
Tuesday's jury verdict concerns a civil case, where Cosby is sued by Judy Huth, 20 years younger.
Cosby has consistently denied the allegations, but admitted that he met Huth during a filming in April 1975 and that he took her and a schoolmate to Playboy Mansion a few days later.
According to Huth, Cosby tried to stick her hand inside her pants.
He is also said to have exposed herself and forced her to perform a sexual act.
Huth sued Cosby as early as 2014. She has said that she was reminded of the traumatic experience when a number of other women came forward and accused Cosby of sexual abuse, and also of her son's 15th birthday.
Cosby himself was not present during the trial.