An air filter for a ventilation system of a motor vehicle has a filter frame and a filter medium fastened in and surrounded by the filter frame. The air filter can be inserted along its two-dimensional extent into a receiving slot and is sealed, at least along the inserted regions of the filter frame, between the filter frame and walls of the insertion slot. To allow the filter to be installed in the ventilation system in as force-free and simple a manner as possible, the filter frame has, in the end side facing the insertion direction, at least one guide slot which is oriented in the insertion direction.