It happened about 10:20 p.m. Tuesday in the 2000 block of Primrose Avenue.
Police said the girl was dressing with her window blinds partially closed when she saw a man standing outside her bedroom window taking a picture.
He's described as white, in his early to mid 30s, standing 6 foot 2 and about 165 pounds. The girl said he had a receding gray hair line, blue eyes, with wrinkle lines on his forehead.
Anyone with information is asked to call South Pasadena police at (626) 403-7280.