The 17-year-old told police she thought she was being followed by the man, who she noticed had been watching her at the Thomas Jefferson Library on Nov. 6.
To curtail the behavior, she ducked into the restroom as she was exiting the building just before 9 p.m., police said. The man followed her into the restroom and the teen ordered him to leave, but he refused, police said.
The suspect then touched and pushed her, but the teen kicked him and screamed, police said. The suspect fled the scene.
The suspect is described as having a black or medium brown complexion. He is 5 feet 7 to 5 feet 10 inches tall and weighs 160 to 180 pounds.
He had a shaven head, brown eyes, a gap in his front teeth and a diagonal scar on his forehead. He wore a pale green, long-sleeved, button-up shirt with khaki pants. He carried a brown or tan soft leather briefcase.
Library staff said he had been there before but had not assaulted other patrons.
Anyone with information is asked to contact Crime Solvers by phone at 1-866-411-TIPS/8477.