Enhancement of leaf photosynthetic capacity can lead to greater biomass productivity in crop plants. Targets for improving leaf photosynthetic capacity in soybean [Glycine max (L.) Merr.], however, remain to be elucidated. The objective of this study was to identify the physiological and morphological factors underlying the diverse photosynthetic capacities of different soybean genotypes. Light-saturated CO 2 assimilation rates ranged from 18.1 to 27.6 μmol m −2 s −1 under controlled conditions among 34 genotypes. PI 594409 A (Line no. 13) and PI 603911 C (Line no. 14) showed extremely high photosynthetic rates. Line no. 14 consistently showed greater photosynthetic rates than other lines under field conditions and reached 34.8 μmol m −2 s −1 , which was 11% greater than that of a reference genotype, Tachinagaha. The analysis of the CO 2 response curve of Line no. 14 showed greater CO 2 fixation activity, represented by the maximum rates of carboxylation (Vc max ) and electron transport (J max ). The leaf ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content of Line no. 14 tended to be higher than that of other lines, which is suggested to contribute to high CO 2 fixation activity. We attribute the high photosynthetic capacity that was observed among soybean genotypes to high CO 2 fixation activity. © Crop Science Society of America.