Los Angeles attorney Walter Phillips would not discuss the specifics of the allegation, which he called "utterly preposterous" — but said it amounts to, at the most, "inappropriate touching."
No charges have been brought against Cosby, but Phillips said authorities have begun an investigation. He said the accuser, who lives in Canada, knows Cosby and the alleged incident in question happened about a year ago.
Police in Cheltenham Township, Pa., a Philadelphia suburb where Cosby has a home, declined to comment.
"I know the person making the accusation hasn't been contacted by authorities," Phillips said in a telephone interview with The Associated Press. "We are hopeful and optimistic that no charges will be brought forward."
A resident of Durham, Ontario, about 25 miles northwest of Toronto, contacted Durham Regional Police about a sexual assault that occurred in the United States, said Dave Selby, spokesman for the Canadian force.
The information was passed on to police in the Philadelphia area, Selby said, but he added, "We're not in a position to identify any of the people who may or may not be involved in it."
Cosby postponed a town hall meeting in Cleveland on Thursday and has postponed three upcoming shows in Florida, his publicist David Brokaw said.
Brokaw would not say whether the postponements have anything to do with the recent allegation.
About 3,600 people, including many public school students and their parents, had received free tickets for the Cleveland event.