Cosby's lawyers had fought the "sexually violent predator" designation, arguing that Pennsylvania's sex-offender law is unconstitutional and that he is no threat to the public at his age. They had also asked the judge to levy a $US25,000 ($35,000) fine on Cosby and make him pay court costs.
Minutes earlier, Constand left court with her arms folded, a smile spreading across her face as other women who have accused Cosby came to hug her.
Constand said she felt "paralyzed and completely helpless" during the assault.
Finding it was appropriate to deem him as such after powerful testimony from the prosecution, Cosby will now be placed under lifetime registration with state police, lifetime sex offender counseling and a community notification will also be given.
More than 60 women subsequently came forward to accuse the once-beloved comedian of a slew of attacks, and in June 2017 he went on trial for assaulting Andrea Constand, a former basketball coach at Temple University in Philadelphia.
Camille Cosby, Bill Cosby's wife, said in a statement that she had retained a former prosecutor to facilitate her efforts to "uncover the truth" regarding what she says is a feud between O'Neill and Castor.
Prosecutors asked for five to 10 years in prison.
Cosby's publicist, Andrew Wyatt, condemned the sentencing.
Cosby is also required to check in with authorities quarterly, according to the AP. He posted $1 million bond and was held under arrest in the weeks leading up to his sentencing.
The Cosby case "really raised awareness of the pervasiveness of ... sexual misconduct against subordinates and against women of relatively less power", said Daniel Filler, dean of Drexel University's law school. Mr. Cosby, this has all circled back to you.
The sentence ensures that Cosby will spend no less than three years in a Pennsylvania prison before he becomes eligible for supervised release, though he could end up staying behind bars for as much as a decade.
The judge ordered Cosby to be immediately taken into custody and officers escorted the handcuffed 81-year-old out of the courthouse.
In her victim statement, Andrea wrote, "Bill Cosby took my lovely, healthy young spirit and crushed it". The defense wants Cosby sent home on house arrest.
"He was good at hiding this for a long time".
Constand spoke briefly on the first day of the hearing on Monday, saying "all I am asking for is justice as the court sees fit". ". Now, nearly 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 move forward".
Bill Cosby victim Andrea Constand, who's led the charge against the disgraced comedian, submitted a victim-impact statement at Cosby's sentencing on Monday. "So it's taken a long time to get there", District Attorney Kevin Steele said. On the first day of the sentencing, the comic laughed at times as the psychologist on the stand for the state portrayed him as a sexual predator with signs of a mental disorder.