An optical instrument, in particular an endoscope, with a hollow instrument housing for receiving optical elements, wherein the distal end and/or proximal end is closed in a fluid-tight manner via an end window. In order to create an optical instrument which, while being easy to assemble, ensures a reliable fluid-tight seal of the instrument housing even during autoclaving, it is proposed according to the invention that the end windows are each sealed off in a fluid-tight manner with respect to the instrument housing via at least one spring-loaded sealing element.