Police responded just after 4 p.m. to the World Academy charter school on the 1700 block of 28th Avenue on a report of an assault, Oakland police Officer Johnna Watson said.
The school had put itself on lockdown before police arrived, she said.
An afterschool program is held at the school and students were still on campus at the time the assault was reported.
Officers arrived and a 7-year-old girl told authorities that she had been approached by a stranger and that he had inappropriately touched her, Watson said.
The stranger had run away from the campus and the student reported the incident to school authorities, Watson said. The girl was not injured in the incident.
Officers searched the area but did not find the suspect, Watson said.
The suspect is described as a white male standing about 6 feet tall and weighing between 180 and 200 pounds.
He has black hair and a mustache and he was wearing a black T-shirt, jeans and brown Sketcher brand shoes.
Anyone with information is asked to call Oakland police at (510) 238-3326.
Staff at the school declined to comment Friday afternoon.