Bill Cosby -- Sued For Allegedly Sexually Assaulting 15-Year-Old

Bill Cosby is being sued by a 56-year-old woman who claims the comedian sexually assaulted her at the Playboy Mansion in 1974 when she was 15 years old ... TMZ has learned.

According to a lawsuit filed at L.A. County Superior Court, Judy Huth claims she and a friend met Cosby on the set of a movie being filmed at a L.A. park and he invited them to his tennis club a week later.

Huth claims when they arrived to the club, Cosby served them booze and played billiards -- requiring them to drink a beer every time Cosby won a game.

According to the suit, Huth claims Cosby then took the girls to the Playboy Mansion and told them to say they were 19 years old if anyone asked.

Huth claims she eventually went to a bedroom with Cosby and that's where she says Cosby began to "sexually molest her by attempting to put his hand down her pants, and then taking her hand in his hand and performing a sex act on himself without her consent."

Huth claims she was emotionally and psychologically damaged from the incident. As for why she filed so late, Huth claims she was so mentally scarred, that she only recently connected her psychological issues with the alleged incident.

We have reached out to Cosby for comment, so far no word back.