The woman, a former Temple University employee who now lives in Canada, came forward in January with accusations about the incident she said happened a year before. Prosecutors last month said they had not found sufficient evidence to support charges against Cosby.
The civil suit was filed Tuesday in U.S. District Court.
"That prosecutor was out of line," said Delores Troiani, an attorney for the woman. "It is not a comment on her credibility, or the evidence in this case."
Cosby has denied the sexual-assault allegations, and in an interview with The National Enquirer last week said, "I am not going to give in to people who try to exploit me because of my celebrity status."
The woman claimed Cosby gave her three blue pills that rendered her semiconscious, and then sexually molested her at his home in suburban Philadelphia in January 2004. She said she awoke to find her bra undone and her clothes in disarray.
According to the suit, the woman also said that after the investigation became public, Cosby's representatives falsely told reporters that her family had asked him for money before going to police. She is seeking defamation damages.
"He has compounded it by making it appear as if she tried to extort money from him," Troiani said.
In the Enquirer interview, Cosby was quoted as saying, "Looking back on it, I realize that words and actions can be misinterpreted by another person."
"I'm not saying that what I did was wrong, but I apologize to my loving wife, who has stood by my side for all these years, for any pain I have caused her," he said. The article did not elaborate.
The long-married Cosby, best known as a warm, wisecracking TV dad, has provoked debate this past year with blunt remarks on personal responsibility aimed at the black community. In 1997, the year his son Ennis was murdered, he acknowledged a brief affair with the mother of Autumn Jackson, a young woman convicted of extorting him.
By David B. Caruso