A day/night weapon sight assembly that consists of daytime sight and a nighttime sight allows the use of both sights without change in eye relief even when the nighttime sight is attached to the rear end of the daytime sight. This is achieved by supporting the daytime sight moveably in the direction of the optical axis and by supporting the nighttime sight with the use of a plane-parallel motion mechanism that allows switching of the nighttime sight between the upper inoperative position and the operative position behind and coaxially with the daytime sight.