The punishment made him the first celebrity of the #MeToo era to be sent to prison and all but completed the dizzying, late-in-life fall from grace for the comedian, former TV star and breaker of racial barriers.
"It is time for justice. Mr Cosby, this has all circled back to you. The time has come," Montgomery County Judge Steven O'Neill said.
He 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, 81, declined the opportunity to speak before the sentence came down, and afterwards 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 in Pennsylvania.
Cosby's lawyers asked that he be allowed to remain free on bail while he appeals against his conviction, but the judge appeared incredulous and ordered him locked up immediately, saying that "he could quite possibly be a danger to the community".
The comedian removed his watch, tie and jacket and walked out in a white dress shirt, his hands cuffed in front of him.
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."
Another Cosby accuser in the courtroom, Lili Bernard, said: "There is solace, absolutely.
"It is his fame and his fortune and his phoney philanthropy that has allowed him to get away with impunity.
"Maybe this will send a message to other powerful perpetrators that they will be caught and punished."
The punishment, which also included a 25,000 dollar (Â£19,000) fine, came at the end of a two-day hearing at which the judge declared Cosby a "sexually violent predator" - a modern-day scarlet letter that subjects him to monthly counselling for the rest of his life and requires that neighbours and schools be notified of his whereabouts.
In a blistering statement, Cosby spokesman Andrew Wyatt said the comic was subjected to the "most racist and sexist trial in the history of the United States".
Among other things, Wyatt said all three of the psychologists who testified against Cosby were "white women who make money off of accusing black men of being sexual predators", and he accused prosecutors of using a doctored recording of a telephone conversation between Constand's mother and Cosby.
At a news conference after the sentencing, Montgomery County district attorney Kevin Steele referred to Cosby's best-known role, saying: "For decades, the defendant has been able to hide his true self and hide his crimes using his fame and fortune. He's hidden behind a character created, Dr Cliff Huxtable."
But "now, finally, Bill Cosby has been unmasked, and we have seen the real man as he is headed off to prison".
Constand stood at Steele's side but shook her head to say she had no comment.
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. It was the first celebrity trial of the #MeToo era.
Cosby faced a sentence of anywhere from probation to 10 years in prison. His lawyers asked for house arrest, saying Cosby, who is legally blind, is too old and vulnerable to do time in prison.
Prosecutors asked for five to 10 years behind bars, saying he could still pose a threat to women.
In court Kevin Steele had rejected the notion that Cosby's age and infirmity entitled him to mercy.
"He was good at hiding this for a long time. Good at suppressing this for a long time. So it's taken a long time to get there," Steele said.
The judge ruled on Cosby's "sexually violent predator" status after a psychologist for the state testified that the entertainer appears to have a mental disorder that gives him an uncontrollable urge to have sex with women without their consent.
When the ruling came down, a woman in the courtroom shot her fist into the air and whispered, "Yessss!"
In a statement submitted to the court and released Tuesday, Constand, 45, said that she has had to cope with years of anxiety and self-doubt. She said she now lives alone with her two dogs and has trouble trusting people.
"When the sexual assault happened, I was a young woman brimming with confidence and looking forward to a future bright with possibilities," she wrote in her five-page statement.
"Now, almost 15 years later, I'm a middle-aged woman who's been stuck in a holding pattern for most of her adult life, unable to heal fully or to move forward."
She also wrote of Cosby: "We may never know the full extent of his double life as a sexual predator, but his decades-long reign of terror as a serial rapist is over."
Constand went to police a year after waking up in a fog at Cosby's gated estate, her clothes askew, only to have the district attorney pass on the case.
Another district attorney reopened the file a decade later and charged the TV star after stand-up comic Hannibal Buress' riff about Cosby being a rapist prompted more accusers to come forward and after a federal judge, acting on a request from The Associated Press, unsealed some of Cosby's startling, decade-old testimony in Constand's related civil suit.
In his testimony, Cosby described sexual encounters with a string of actresses, models and other young women and talked about obtaining quaaludes to give to those he wanted to sleep with.
Cosby's first trial in 2017 ended with a hung jury.
He was convicted at a retrial that opened months after the #MeToo movement had taken down such figures as Hollywood studio boss Harvey Weinstein and actor Kevin Spacey.
Constand said Cosby gave her what she thought were herbal pills to ease stress, then penetrated her with his fingers as she lay immobilised on a couch.
Cosby claimed the encounter was consensual, and his lawyers branded her a "con artist" who framed the comedian to get a big payday - a multimillion dollar settlement she received over a decade ago.
Five other accusers took the stand at the trial as part of an effort by prosecutors to portray him as a predator.
Cosby broke barriers in the 1960s as the first black actor to star in a network show, I Spy. He went on to superstardom as wise and understanding Dr Cliff Huxtable on The Cosby Show, a sitcom that showed America a new kind of black TV family: a warm and loving household led by two professionals, one a lawyer, the other a doctor.
He won a Presidential Medal of Freedom and countless Emmys, Golden Globes and Grammy awards.
As the allegations mounted, his career all but collapsed, Cosby Show reruns were taken off the air, and one college after another stripped him of his honorary degrees.