The teen was walking in the area of Idaho Street and Whittier Boulevard in La Habra between 3 and 3:30 p.m. when she was approached by the man.
He got within arm's reach when she pepper-sprayed him and ran away.
Detectives are looking at surveillance video from the area for more clues to help identify the man.
The suspect is described as a heavy-set white man with a beard, approximately 40 to 50 years old and last seen wearing a dark baseball hat and dark jacket.
Anyone with information is urged to call the La Habra Police Department at (562) 383-4300.