An American student who sexually assaulted a woman will avoid prison, a judge has ruled.

Austin James Wilkerson, 22, was convicted for assaulting a "helpless" woman a year his junior after she reportedly refused to go along with his sexual advances.

Despite a potential prison sentence of up to 12 years for the offence, the judge ruled that Wilkerson would instead serve two years of "work release" and 20 years to life on probation.

The court heard that Wilkerson "isolated and raped the half-conscious victim" after telling friends he would look after her.

He was said to have "digitally and orally penetrated" her after "not getting much response".

Prosecutors added that the rejections “pissed off” Wilkerson and that he called her a “f*****g b***h.”

The woman reportedly told the court she suffered from panic attacks and nightmares after the assault, adding that her own mother partially blamed her for the attack.

The judge's ruling means Wilkerson, who has been suspended from the University of Colorado, will spend his nights at a county jail but will be able to work or go to school during the day.

Boulder District Judge Patrick Butler reportedly told the court: “I’ve struggled, to be quite frank, with the idea of, ‘Do I put him in prison?’

“I don’t know that there is any great result for anybody. Mr. Wilkerson deserves to be punished, but I think we all need to find out whether he truly can or cannot be rehabilitated.”

The decision not to sentence Wilkerson to prison has sparked an angry reaction on social media, with users saying they are "utterly disgusted" at the ruling.