The punishment handed down all but completed the dizzying, late-in-life fall from grace for the comedian, former TV star and breaker of racial barriers, who was led from the court in handcuffs.
Cosby is the first celebrity of the “MeToo” era to be sent to prison.
“It is time for justice,” Montgomery County Judge Steven O’Neill said in sentencing on Tuesday.
“Mr Cosby, this has all circled back to you. The time has come.”
The judge quoted from victim Andrea Constand’s own statement to the court, in which she said Cosby took her “beautiful, young spirit and crushed it”.
Cosby declined the opportunity to speak before the sentence came down, and afterward he sat smiling, laughing and chatting with his defence team.
His wife of 54 years, Camille, was not in court.
Constand smiled broadly upon hearing the punishment and was hugged by others in the courtroom.
Cosby’s lawyers asked that he be allowed to remain free on bail while he appeals his conviction, but the judge appeared incredulous over the request and turned it down.
The judge said that even considering Cosby’s age and blindness, “he could quite possibly be a danger to the community”.
Cosby was also fined $US25,000 ($A34,445).
Former model Janice Dickinson, who was among the 60 or so women who have come forward to accuse Cosby of drugging and violating them over the past five decades, looked at Cosby and said, “here’s the last laugh pal”.
The punishment came at the end of a two-day hearing at which the judge declared Cosby a “sexually violent predator” – a ruling that subjects him to monthly counselling for the rest of his life and requires that neighbours and schools be notified of his whereabouts.
The comic once known as “America’s Dad” for his role on the top-rated Cosby Show in the 1980s was convicted in April of violating Constand, Temple University women’s basketball administrator, at his suburban Philadelphia estate in 2004.