Provided is a seal ring which can reduce the amount of leakage of fluid to be sealed, the leakage flowing from a high-pressure region to a low-pressure region. A seal ring (100) is mounted in an annular groove (210) provided in the outer periphery of a shaft (200). The seal ring (100) seals the annular gap between the shaft (200) and a housing (300), the shaft (200) and the housing (300) rotating relative to each other, and separates a low-pressure region (L) and a high-pressure region (H) in which fluid to be sealed is present. When the shaft (200) and the housing (300) rotate relative to each other, a side surface of the seal ring (100), the side surface facing the low-pressure region, and a side surface of the annular groove slide on each other. The seal ring (100) is provided with a side surface groove (110) which is provided in a side surface of the seal ring (100), the side surface facing the low-pressure region (L), and which extends from the position at which the side surface groove (110) is exposed to the low-pressure region (L), toward the inner peripheral surface side of the seal ring (100) to a position not reaching the inner peripheral surface.