A reflex sight for a firearm has an optical element received within a receptacle of a housing. The housing is mounted on the firearm. Shoulders extending from a base of the optical element engage corresponding shoulders extending from the housing into the receptacle to position the optical element so that it intersects a line of sight parallel to but offset from the firing axis of the firearm. The optical element includes a collimating reflector which produces a virtual reticle visible when viewed along the line of sight when aiming the firearm. The optical element is captured within the receptacle between the firearm or a plate and the shoulders of the housing.