A gas sensor, including a cylindrical outer tube; and a seal member disposed therein, the seal member including: a lead wire insert hole and an atmosphere communication hole. The seal member has a groove extending outwards in a radial direction at a rear end-facing surface from the atmosphere communication hole while circumventing the lead wire insert hole, the groove having a cutout toward the front end of the seal member. The gas sensor further includes: a protection portion which covers the atmosphere communication hole; and an arm couples the protection portion and the outer tube, and extends in the radial direction, the arm being at least partially disposed in the groove of the seal member. and the arm is disposed so as to define a gap with the bottom surface of the groove.