A rotary turbine bypass valve comprising: a valve chamber positioned at a junction of an inlet port, an outlet port and a bypass port, the inlet port configured for fluid communication with a flow of exhaust gas from an engine, the outlet port configured for fluid communication with an inlet of a turbine, and the bypass port configured for fluid communication with an exhaust aftertreatment device; and a valve rotor supported for rotation, about a valve axis, within the valve chamber; wherein the valve rotor is rotatable about the valve axis between a first position in which the valve rotor permits gas flow through the bypass port and a second position in which the valve rotor blocks gas flow through the bypass port; wherein the valve rotor is eccentric such that it includes a seal portion which is a portion of the valve rotor within the valve chamber which is furthest radially spaced from the valve axis; and as the valve rotor moves from the first position to the second position the seal portion moves towards the bypass port such that the radial separation between the valve rotor and the bypass port decreases to a minimum when the valve rotor is in the second position in which the seal portion is adjacent the bypass port; wherein the seal portion comprises a rotor seal feature sized and shaped to co-operate with the bypass port to plug the bypass port when the valve rotor is in the second position; and wherein the rotor seal feature comprises a raised sealing face which has a shape corresponding to that of an opening of the bypass port, the sealing face being received by the bypass port when the valve rotor is in the second position. a valve chamber positioned at a junction of an inlet port, an outlet port and a bypass port, the inlet port configured for fluid communication with a flow of exhaust gas from an engine, the outlet port configured for fluid communication with an inlet of a turbine, and the bypass port configured for fluid communication with an exhaust aftertreatment device; and a valve rotor supported for rotation, about a valve axis, within the valve chamber; wherein the valve rotor is rotatable about the valve axis between a first position in which the valve rotor permits gas flow through the bypass port and a second position in which the valve rotor blocks gas flow through the bypass port; wherein the valve rotor is eccentric such that it includes a seal portion which is a portion of the valve rotor within the valve chamber which is furthest radially spaced from the valve axis; and as the valve rotor moves from the first position to the second position the seal portion moves towards the bypass port such that the radial separation between the valve rotor and the bypass port decreases to a minimum when the valve rotor is in the second position in which the seal portion is adjacent the bypass port; wherein the seal portion comprises a rotor seal feature sized and shaped to co-operate with the bypass port to plug the bypass port when the valve rotor is in the second position; and wherein the rotor seal feature comprises a raised sealing face which has a shape corresponding to that of an opening of the bypass port, the sealing face being received by the bypass port when the valve rotor is in the second position.