This paper reports on a basin-wide inventory of anthropogenic CO2 in the East (Japan) Sea determined from high-quality alkalinity, chlorofluorocarbon, and nutrient data collected during a summertime survey in 1999 and total dissolved inorganic carbon data calculated from pH and alkalinity measurements. The data set comprises measurements from 203 hydrographic stations and covers most of the East Sea with the exception of the northwestern boundary region. Anthropogenic CO2 concentrations are estimated by separating this value from total dissolved inorganic carbon using a tracer-based (chlorofluorocarbon) separation technique. Wintertime surface CFC-12 data collected in regions of deep water formation off Vladivostok, Russia, improve the accuracy of estimates of anthropogenic CO2 concentrations by providing improved air-sea CO2 disequilibrium values for intermediate and deep waters. Our calculation yields a total anthropogenic CO2 inventory in the East Sea of 0.40 ± 0.06 petagrams of carbon as of 1999. Anthropogenic CO2 has already reached the bottom of the East Sea, largely owing to the effective transport of anthropogenic CO2 from the surface to the ocean interior via deep water formation in the waters off Vladivostok. The highest specific column inventory (vertically integrated inventory per square meter) of anthropogenic CO2 of 80 mol C m−2 is found in the Japan Basin (40°N−44°N). Comparison of this inventory with those for other major basins of the same latitude band reveal that the East Sea values are much higher than the inventory for the Pacific Ocean (20−30 mol C m−2) and are similar to the inventory for the North Atlantic (66−72 mol C m−2). The substantial accumulation of anthropogenic CO2 in the East Sea during the industrial era has caused the aragonite and calcite saturation horizons to move upward by 80−220 m and 500−700 m, respectively. These upward movements are approximately 5 times greater than those found in the North Pacific. Both the large accumulation of anthropogenic CO2 and its significant impact on carbonate chemistry in the East Sea suggest that this sea is an important site for monitoring the future impact of the oceanic invasion of anthropogenic CO2.