A boss for a pressure vessel, comprising: a boss body in which a through hole configured to connect an interior to an exterior of the pressure vessel is formed; and a boss joint portion which surrounds an outer surface of the boss body, radially extends outward, and has an outer surface coming into surface contact with and joined to an opening surface of a liner, wherein the outer surface of the boss joint portion has a cross-sectional shape having three or more line segments connecting a top to a bottom of the boss joint portion and including at least one convex corner and at least one concave corner, wherein the outer surface of the boss joint portion comprises a stepwise cross section including a first side part on a lower side of the boss joint portion, a second side part on an upper side of the boss joint portion spaced apart from the first side part, and a stepped surface connecting the first side part and the second side part to each other, characterized in that: each of the first side part and the second side part is formed to be tapered, the first side part and the second side part are tapered in opposite directions, the first side part has a greater separation distance from a central line than the second side part, the first side part is tapered in a shape in which a radial distance from the central line is gradually reduced toward a bottom of the boss joint portion, and the second side part is tapered in a shape in which a radial distance from the central line is gradually reduced toward a top of the boss joint portion. a boss body in which a through hole configured to connect an interior to an exterior of the pressure vessel is formed; and a boss joint portion which surrounds an outer surface of the boss body, radially extends outward, and has an outer surface coming into surface contact with and joined to an opening surface of a liner, wherein the outer surface of the boss joint portion has a cross-sectional shape having three or more line segments connecting a top to a bottom of the boss joint portion and including at least one convex corner and at least one concave corner, wherein the outer surface of the boss joint portion comprises a stepwise cross section including a first side part on a lower side of the boss joint portion, a second side part on an upper side of the boss joint portion spaced apart from the first side part, and a stepped surface connecting the first side part and the second side part to each other, characterized in that: each of the first side part and the second side part is formed to be tapered, the first side part and the second side part are tapered in opposite directions, the first side part has a greater separation distance from a central line than the second side part, the first side part is tapered in a shape in which a radial distance from the central line is gradually reduced toward a bottom of the boss joint portion, and the second side part is tapered in a shape in which a radial distance from the central line is gradually reduced toward a top of the boss joint portion. each of the first side part and the second side part is formed to be tapered, the first side part and the second side part are tapered in opposite directions, the first side part has a greater separation distance from a central line than the second side part, the first side part is tapered in a shape in which a radial distance from the central line is gradually reduced toward a bottom of the boss joint portion, and the second side part is tapered in a shape in which a radial distance from the central line is gradually reduced toward a top of the boss joint portion.