An air conditioner including a main body including a first side, a second side opposite to the first side, an outlet between the first and second side, and a guide protrusion on the outlet; and a blade to open and close the outlet, the blade including a blade body extending from the first side to the second side of the main body when the outlet is closed by the blade, and a guide rail on a top surface of the blade body facing the outlet. The blade is configured so that the blade body is rotatable relative to the outlet to adjust a direction of air from the outlet, and the guide rail is supported by the guide protrusion to be slidable to guide at least one of a translational or a rotational movement of the blade body with respect to the main body during rotation of the blade body.