A gas sensor includes a sensing element, a metal shell that is tubular and that surrounds the sensing element, and a protector made of a metal and fixed to the metal shell. A back end portion of the protector is joined to an outer surface of the metal shell to form a joined portion. A coefficient of thermal expansion of a material of the protector at 800 degrees Celsius is higher than a coefficient of thermal expansion of a material of the metal shell at 800 degrees Celsius. In a cross section of a portion of the gas sensor including the joined portion, a minimum distance t1 between the outer surface of the metal shell and an outer surface of the protector and a minimum distance t2 between the outer surface of the metal shell and an inner surface of the metal shell satisfy 0.6≤(t1/t2)≤2.0.