Thermometer guard is an antiseptic guard for a clinical thermometer. It is formed by a sheath constituted by two synthetic resin films welded together along two lateral edges and a top edge connecting the lateral edges. A welded joint is formed at an end of the sheath spaced from the top edge to hold the tip of the thermometer in position while a bendable flap is formed between the joint and the top edge. When the sheathed thermometer is inserted in the anus, the flap is folded back and exposes smooth surfaces of the films, avoiding direct contact with weld edges and thus preventing formation of sores.