The shooting was reported just after 6 p.m. in the 900 block of North Raymond Avenue in Pasadena.
The boy was apparently playing video games in his bedroom when three to five shots were fired nearby and at least one bullet came in through the window.
Police say first responders arrived to the home and found the teen there with a gunshot wound. They performed CPR and transported him to a local hospital, where he was declared dead.
Details about the bullet's origin and the intended target were still under investigation.
Pasadena police are asking the public to come forward with any information.
"If you know anything about this tragic incident, or you saw anything, please, please, please come forward to the Pasadena Police Department," said police Cmdr. Mark Goodman. "We will keep your information anonymous. What we want to do is find the people who are responsible for this tragic incident and bring them to justice."