We present climatological carbon dioxide (CO<sub>2</sub>) distributions over the Asia-Pacific region obtained from the CONTRAIL (Comprehensive Observation Network for Trace gases by Airliner) measurements. The high-frequency in-flight CO<sub>2</sub> measurements over 10 years reveal a clear seasonal variation of CO<sub>2</sub> in the upper troposphere (UT), with a maximum occurring in April–May and a minimum in August–September. The CO<sub>2</sub> mole fraction in the UT north of 40° N is low and highly variable in June–August due to the arrival of air parcels with seasonally low CO<sub>2</sub> caused by the summertime biospheric uptake in boreal Eurasia. For August–September in particular, the UT CO<sub>2</sub> is noticeably low within the Asian summer monsoon anticyclone associated with the convective transport of strong biospheric CO<sub>2</sub> uptake signal over South Asia. During September as the anticyclone decays, a spreading of this low CO<sub>2</sub> area in the UT is observed in the vertical profiles of CO<sub>2</sub> over the Pacific Rim of the continental East Asia. Simulation results identify the influence of anthropogenic and biospheric CO<sub>2</sub> fluxes in the seasonal evolution of the spatial CO<sub>2</sub> distribution over the Asia-Pacific region. It is found, for example, that a substantial contribution to the UT CO<sub>2</sub> over the northwestern Pacific comes from the continental East Asian emissions in the spring, but switches to South Asian and/or Southeast Asian air masses affected dominantly by the biospheric CO<sub>2</sub> uptake in the summer monsoon season. The CONTRAIL CO<sub>2</sub> data provide useful constraints to model estimates of surface fluxes and to the evaluation of the satellite observations, in particular for the Asia-Pacific region.