They cited the statute of limitations -- which limits how long legal action can be taken after an alleged crime -- in declining to pursue the case, brought by former Playboy hostess Judy Huth earlier this month.
About 20 women have accused the 77-year-old Emmy and Golden Globe winner of sexual assault and even rape in alleged incidents dating mostly from decades ago.
But so far the only one who had taken legal action was Huth, who was represented by celebrity lawyer Gloria Allred.
"Given that 40 years have passed since the alleged incident, prosecution (on a misdemeanor count) ... is declined due to the expiration of the statute of limitations," wrote Deputy District Attorney Victor Rodriguez.
Allred did not immediately respond to a request for reaction to the prosecutor's decision.
Cosby is famous for his role in "The Cosby Show," a popular family sitcom that aired in the 1980s and 1990s.