Renita Hill filed the claim saying she had been “emboldened” by the actions of Mr Cosby’s other alleged victims.
Seven women are suing him, claiming he portrayed them as liars when they went public with their accusations.
Mr Cosby filed a counterclaim, alleging their claims were only for financial gain.
The 78-year-old is also seeking unspecified monetary damages and said the women inflicted emotional distress.
Ms Hill’s defamation case said she was made out to be a “liar” and “extortionist” when the comedian, his wife and his lawyer issued blanket denials.
But judge Arthur Schwab agreed with Mr Cosby’s legal representatives that the statements were opinions protected by the First Amendment.
‘Innuendo and undisclosed facts’
“Even considering these three statements together as a combined, single statement, this newly ‘conjoined’ statement does not lead to an inference that the plaintiff is a ‘liar and an extortionist’,” he said.
Ms Hill’s legal representative, George Kontos, said he would be appealing the decision, adding he strongly disagreed with the judge’s reasoning.
“The basis of his opinion is these are constitutionally protected, ‘pure’ opinions,” said Mr Kontos.
“But, looked at as a whole, they contain all sorts of innuendo and undisclosed facts.”
Mr Cosby has not responded specifically to Ms Hill’s allegations that he drugged and assaulted her several times after they met on the TV show Picture Pages in 1983.
She claims she was 16 at the time and the abuse continued for four years. She also says Mr Cosby helped pay for her college fees and travel to cities where he was performing.
Mr Cosby has repeatedly denied all accusations against him made by more than 50 women.
He has, however, admitted in court proceedings that he obtained and gave women sedatives over the years, but he maintained the women took the drugs willingly.
At the end of 2015, prosecutors in Pennsylvania charged the comedian with an alleged sexual assault in 2004. It was the first time Mr Cosby had been charged with any offence after months of accusations.
He is currently on $1m (£675,000) bail.