Selection for cultivars with superior responsiveness to elevated atmospheric CO2 concentrations (eCO2) is a powerful option for boosting crop productivity under future eCO2. However, neither criteria for eCO2 responsiveness nor prescreening methods have been established. The purpose of this study was to identify traits responsible for eCO2 responsiveness of soybean (Glycine max). We grew 12 Japanese and U.S. soybean cultivars that differed in their maturity group and determinacy under ambient CO2 and eCO2 for 2 years in temperature gradient chambers. CO2 elevation significantly increased seed yield per plant, and the magnitude varied widely among the cultivars (from 0% to 62%). The yield increase was best explained by increased aboveground biomass and pod number per plant. These results suggest that the plasticity of pod production under eCO2 results from biomass enhancement, and would therefore be a key factor in the yield response to eCO2, a resource-rich environment. To test this hypothesis, we grew the same cultivars at low planting density, a resource-rich environment that improved the light and nutrient supplies by minimizing competition. Low planting density significantly increased seed yield per plant, and the magnitude ranged from 5% to 105% among the cultivars owing to increased biomass and pod number per plant. The yield increase due to low-density planting was significantly positively correlated with the eCO2 response in both years. These results confirm our hypothesis and suggest that high plasticity of biomass and pod production at a low planting density reveals suitable parameters for breeding to maximize soybean yield under eCO2.