The objective of this study was to elucidate variability among soybean cultivars in yield response at different planting densities in reference to branch development. We investigated the main stem and branch seed yield and the branching characteristics of determinate Hokkaido and indeterminate US cultivars at the Rakuno Gakuen University in Ebetsu. In 2009 and 2010, two Japanese and two US cultivars were grown at three densities from 9.5 to 20 plants m(-2). In 2011 and 2012, three cultivars from each region were cultivated at three densities from 8.3 to 22.2 plants m(-2). The seed yields of the US cultivars at densities of 16.7 plants m(-2) or less were markedly higher than those of the Hokkaido cultivars, showing that their yield is less sensitive to lower planting density than Hokkaido cultivars. The difference in yield in response to planting density among cultivars was closely associated with a larger increase in branch seed yield with lower planting density, which effectively compensated for the decrease in main stem number per unit land area. The variability of branch development in response to planting density (branching plasticity) was quantified by correlating branch performance with plant spacing (land area per plant). Some US cultivars exhibited greater branching plasticity than Japanese cultivars with similar growth duration. Results of this study suggest soybean cultivar differs in responsiveness to varied planting density through different branching plasticity.