Ozonesonde data collected over the western Pacific in the boreal summer of 2021, which is the first continuous, direct ozone observation over the region, were used to investigate the material transport and dynamical variations associated with the summer monsoon and western Pacific convection. The data revealed ozone transport under convectively calm conditions over the western Pacific during the development phase of the Asian summer monsoon. Combining atmospheric reanalysis data and satellite cloud data, the transport process of a high-ozone air mass is described. Near the date line, the air mass intruded from the midlatitude stratosphere into the tropical upper troposphere due to Rossby wave breakings, then moved westward over the Pacific Ocean along 20°N, experiencing nearly adiabatic transport. Subsequently, as it approached the convectively active region of Southeast Asia, it underwent non-conservative mixing with the tropospheric air due to convective processes. The climatological situation in the summer Asia-Pacific region can facilitate frequent occurrences of this type of transport, which may play a role in determining ozone concentration in the tropical upper troposphere.