Under the law, if someone says "no" at any time the other person must stop or it becomes rape. The National Crime Victim Law Institute said it believed the law is the first of its kind in the country.
Lyn Schollett, general counsel for the Illinois Coalition Against Sexual Assault, said the law was important to make it clear to victims, offenders, prosecutors and juries that people have the right to halt sexual activity at any time.
"I think it will empower prosecutors in charging cases where the victim and the offender have a sexual history," she said.
But the director of the Victim Advocacy & Research Group in Boston said it would be hard to imagine courts not upholding a woman's right to withdraw consent.
"To me, it's demeaning," Wendy Murphy said. "It's like the old saying: 'If it ain't broke, don't fix it.' I don't think it was broke."
The law was inspired by a California case involving two 17-year-olds who had sex at a party. The girl changed her mind about having sex, but the boy did not stop immediately.
He was charged with rape, and it took years for the courts to decide that he could be found guilty under California law. The California Supreme Court ruled in January that a man can be convicted if a woman first consents but later asks him to stop.
Lawmakers said they wanted to avoid the same kind of long legal battle in Illinois. Gov. Rod Blagojevich signed the law Friday but did not announce it until Monday.