The Pennsylvania judge made the ruling ahead of sentencing the comedian for drugging and molesting a woman at his suburban Philadelphia home in 2004, reports bbc.com.
At a retrial in April, Cosby was found guilty of three counts of sexual assault against Andrea Constand.
The classification by court means he will need to register with state police and notify any community he lives in of his sex offender status, as well as undergo mandatory counselling for life. Neighbours, childcare centres and schools will have be notified of his whereabouts.
The actor’s defence team had argued the state’s sex offender law was too severe given Cosby’s age, as well as the fact that he is legally blind.
The two-day hearing at Montgomery County Courthouse in Pennsylvania began on Monday.
Psychologist Kristen Dudley testified on Monday that Cosby showed signs of a mental disorder and was likely to reoffend.
Apart from the label, Cosby still faces a maximum prison term of 10 years after an agreement between prosecutors and his defence team, but there is no mandatory minimum for the convictions.
Montgomery County District Attorney Kevin Steele called for five to 10 years in jail for aggravated indecent assault.
Cosby’s defence is pushing instead for house arrest, according to bbc.com.
In June 2017, Constand described how Cosby, whom she viewed as a “mentor”, gave her pills that left her “frozen” and unable to stop his assault.
The comedian was arrested in 2015 and a deadlocked jury resulted in a mistrial in June 2017.
This year’s retrial occurred amid the #MeToo movement that has seen people worldwide come forward to share stories of sexual harassment and assault.