Brock Turner, 21, who was a swimming champion, assaulted the woman outside a Stanford University fraternity house.
He was freed from the Santa Clara County Main Jail in San Jose, California, after serving three months.
Inmates in California typically serve 50% of their sentences before release.
Turner, who left the jail without commenting to journalists, apparently intends to return to Ohio to live with his parents.
Under the terms of his release, he faces three years of supervised probation and will be registered as a sex offender for the rest of his life.
The local sheriff's office in Ohio said Turner would be treated "no differently than any other sex offender we monitor".
The former swimmer's sentence prompted an outcry, with critics saying the judge had been overly lenient and sex assaults on US campuses were not being treated seriously enough.
Prosecutors had demanded a sentence of six years in state prison.
Judge Aaron Persky, who handed down the sentence, expressed concern about the impact of prison on Turner. In his decision, he said positive character references and Turner's remorse and lack of previous criminal record were mitigating factors.
Turner's father, Dan, also sparked outrage during the case, saying his son should not have been jailed for "20 minutes of action".
Two witnesses said they had seen him sexually assaulting the woman, who was lying on the ground unconscious.
The victim, referred to by the pseudonym Emily Doe, released her victim impact statement publicly. It was read by millions, and praised by US Vice President Joe Biden.
She criticised Turner for not admitting the crime and putting her and her family through an "invasive" and "aggressive" trial.
Later, she expressed disappointment at the "gentle" sentence her attacker received.
California attorney general Kamala Harris added her voice to the criticism, questioning whether the facts "merit that kind of mitigation".
Protesters have called for the recall of Judge Persky, who last month asked to be assigned to the court's civil division.
Earlier this week, legislators in California closed a loophole in sexual assault cases, whereby more lenient sentences could be issued if the victim did not resist.
In California, the use of force in a sexual assault results in a mandatory prison sentence. In cases where no force is used - when the victim is unable to defend themselves - no mandatory sentence exists.
State assembly members voted unanimously to amend the law, and prevent the use of probation in such cases. The bill has been passed to Governor Jerry Brown for approval, but has not yet been signed into law.
A student at a top US university whose six-month jail sentence for sexually assaulting an unconscious woman in 2015 was widely criticised for being too lenient has been released from prison.