A seal ring and a sealing structure that reduce the deviation of the central axis of the seal ring from the central axis of a shaft thereby preventing the seal ring from abutting against the tip end of a housing in the operation of inserting the shaft in the housing. The seal ring includes a thin part 100A provided in a range less than 180° with respect to a circumferential direction and a thick part 100B, which is configured to protrude inward in the radial direction from an inner peripheral surface of the thin part 100A, the thin part 100A has an abutment joint part 110, and the thick part 100B has, at respective ends of both side surfaces on an inner peripheral surface side thereof, a concave part 120 over an entire range thereof to form a gap between the ends of the both side surfaces and corner parts at both ends of a groove bottom of the annular groove.