A knife and sheath assembly includes a sheath including a knife receptacle defining knife-receiving open chamber, a knife stopper located at the knife receptacle and a stretchable part operable to move said knife stopper, a knife including a blade insertable into the knife-receiving open chamber, a handle and a shoulder connected between the blade and the handle and stoppable by the knife stopper to keep the blade in the knife-receiving open chamber, and an operating member including a mounting part affixed to the sheath or the knife and an actuating part connected to the mounting part and operable to force the stretchable part of the sheath against the knife stopper in direction away from the shoulder of the knife.