American comedian and actor Bill Cosby has been charged with aggravated indecent assault, the first formal accusation to arise from the abuse claims of more than 50 women, some dating back decades.

Mr Cosby, 78, briefly appeared before a Pennsylvanian court on Thursday morning, where he was freed after paying $US1 million bail. He is yet to enter a plea.

“Today, after examination of all the evidence, we are able to seek justice on behalf of the victim,” district attorney Kevin Steele said at a press conference. Mr Steele was recently elected to the role, one of his platforms included a promise to prosecute Cosby.

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Police will allege the entertainer drugged a woman, Andrea Constand, when she was a guest at his Philadelphia-area mansion in January 2004 and sexually assaulted her when she was incapable of consenting or resisting.

“The victim did not consent to any of these acts and reported that she was unable to move or speak and felt frozen and paralysed,” prosecutor Mr Steele said.

Mr Cosby previously settled a civil case with the alleged victim, Ms Constand.

The single charge was laid just before the statute of limitations expired in January. It carries a maximum penalty of 10 years in prison.

More than 50 women have accused Cosby, best known for playing Dr Cliff Huxtable in the sitcom The Cosby Show, of sexually assaulting them without their consent, usually while they were allegedly drugged, with most incidents dating back decades to the height of his career in Hollywood.

The entertainer has repeatedly denied all allegations.

-with AAP and ABC