The Advanced Combat Optical Gunsight (abbreviated ACOG) is a series of telescopic sights manufactured by Trijicon. The ACOG is designed to be used on the M16 rifle and M4 carbine, but Trijicon has developed ACOG accessories for certain other firearms. Models provide magnification levels between 1.5x and 6x.

Unlike many reflex sights, the ACOG does not require battery power. The superimposed reticle, available in a variety of patterns and colors, is illuminated by an external fiber optic light-gathering rail when available light is plentiful, and by an internal tritium lamp during low-light use.