Juliet Nunez was last seen leaving her home around 1 p.m. on Sept. 18, according to the Missing People in America website.
The teenager is on psychiatric medication and could be experiencing a manic episode.
She is described as being 5 feet 6 inches, weighing about 135 pounds. Nunez was last seen wearing a hoodie with leggings. She also has a nose ring.
She is known to frequent parks and stores in the area and has been spotted in Redlands, Rialto and Fontana.
Anyone with information is urged to call police at (909) 820-2550.