Medium-sized rings are important structural units, but their synthesis, especially in a highly enantioselective manner, has been a great challenge. Herein we report an enantioselective synthesis of medium-sized-ring lactones by an iridium-catalyzed Z-retentive asymmetric allylic substitution reaction. The reaction features mild conditions and a broad substrate scope. Various eight- to 11-membered-ring lactones can be afforded in moderate to excellent yields (up to 88%) and excellent enantioselectivity (up to 99% ee). The utilization of both Z-allyl precursors and an Ir catalyst is critical for the medium-sized-ring formation.