Abstract: A knife valve comprises a body defining a valve seat and a knife blade longitudinally movable by a cylindrical stem in the body for opening and closing a lateral flow passage defined therethrough. The body includes a bonnet with the stem extending therethrough and with circular packing rings being disposed therein around the stem. The valve seat is located on a first side of the blade and in operative proximity thereto. The stem includes a lower head portion which defines an angular surface, whereas a top portion of the blade defines a slot forming an angular surface parallel to the angular surface of the stem head. The stem head is engageable in the slot with the angular surfaces thereof being in sliding contact. Therefore, when the blade is in its closed position, a longitudinal force exerted by the stem on the blade is transferred into a lateral force oriented towards the seat, thereby forcing the blade against the seat in a tight bidirectional seal under a positive torque.