The Canadian Broadcasting Corp. said more than a dozen women have been attacked from behind as they walked at night in the past several months.
"We believe we have the person responsible for these assaults," Toronto police Chief Bill Blair said. "These [are] crimes that ... caused a tremendous amount of fear within our community.
"Fear, particularly among women, and their sense of safety in their own neighborhood was taken from them as a result of these crimes."
Toronto police said the teenager, whose name was not released, was arrested about 11 p.m. Saturday at the scene of an alleged sexual assault.
Blair said the case was broken thanks to young female undercover police officers.
"We had women in our organization who put themselves in harm's way, who went out in that community and put themselves at risk, knowing that if they were successful in their mission, they could also be the victim of an assault," Blair said.