Mr Cosby befriended the woman in Philadelphia. Photo: AP
Prosecutors in Pennsylvania have filed the first sexual assault charges against US comedian Bill Cosby.
Mr Cosby, 78, has been charged with aggravated indecent assault over an alleged incident in 2004 involving a former local university employee.
He has previously said under oath he had a consensual sexual encounter with the woman. The case was re-opened after dozens of women alleged assault by Mr Cosby, dating back to the 1970s.
Andrea Constand, who was an employee at Temple University at the time of the alleged assault, sued Mr Cosby in 2005 and settled for an undisclosed sum after no charges were brought.
She has said she was tricked into taking drugs before being sexually assaulted by Mr Cosby. Mr Cosby has repeatedly denied any wrongdoing in relation to the allegations.
Documents from the case were sealed until this summer.
Testimony from the lawsuits include Mr Cosby admitting he gave women a sedative, Quaaludes, when he wanted to have sex with them.
Mr Cosby befriended Ms Constand through her job at the university and she came to think of him as "a mentor and a friend" before he allegedly made two sexual advances that were rejected, said prosecutors.
Kevin Steele, district attorney for Montgomery County, said they re-visited the case after new evidence emerged, with the statue of limitations not expired yet.
The prosecutors re-interviewed witnesses, examined evidence from the civil case and spoke to other alleged victims before they decided to file criminal charges, Mr Steele's office says in a statement.
At the height of his fame from The Cosby Show, the comedian was the highest paid entertainer on US television.