The shooting happened at about 7 a.m. at a Western Forest Products sawmill in Nanaimo, north of Victoria.
The RCMP say officers arrested a 47-year-old resident of Nanaimo shortly after arriving at the scene.
Western Forest Products president Don Demens says the suspect is a former employee, but the man's name hasn't been publicly released.
Supt. Mark Fisher of the RCMP says officers seized a shotgun at the scene.
The RCMP have not speculated on what may have motivated the attack, but Fisher says investigators believe the shooter worked alone.