The young teen "was loitering suspiciously in the back yard of a residence along Russellwood Avenue," McKees Rocks police say.
Following an investigation, he was found to have stolen a 9mm gun, according to the release.
Three bricks of heroin and the loaded gun were seized from the boy when he was taken into custody.
Additional information was unavailable as the incident involves a minor.
Click here to follow Daily Voice Lower Merion-Narberth and receive free news updates.