Cops are investigating Justin Bieber on an assault charge -- with a Nerf gun.

The trouble started in November when a female employee of an Ottawa, Canada venue where Justin was performing claimed the singer or one of his siblings assaulted her with a foam dart. According to TMZ, Justin was Nerf fighting with his three-year-old brother and four-year-old sister, and allegedly one of the kid-safe projectiles hit the woman. She was so angry that she filed a police report -- and the cops are taking it seriously, having already interviewed witnesses at the venue.