Abstract: The self-contained day/night optical sight device of the invention has a sealed sight housing permanently attached to the weapon or to other object and containing an objective lens and an eyepiece lens installed on a common optical path at a distance from each other so that a space is formed between the both. The same sealed housing pivotally supports a night-vision unit, such as an image-intensifier tube, which can be turned in the plane that contains the optical axis of the sight between the position offset from the aforementioned common optical axis and the position coincident with this optical axis. Since both the night-vision and day-vision optics are located in a sealed housing, the lenses are protected from contamination and fogging. The use of a single optical path makes it possible to reduce the weight of the system.