A stern tube sealing assembly and a seal ring thereof has a key part having an inverted V-shape, a front step part on the front side, and a back step part on the back side thereof. The seal ring width in a propulsion-shaft axis direction becomes thinner in the transition area from the key part toward a heel part. The inner diameter of the key part at the back step part is greater than that of the key part at the front step part. The ring has a lip part having a V-shaped lip tip protruding inwardly in the radial direction and a spring groove having a semicircular cross-section. The center of the spring groove is nearer to the inboard side than the lip tip, by an offset in the axis direction, the offset being within approximately 10% of the orthogonal projection length of the width of the lip back.