The rate of growth of individual seeds of soybean (Glycine max L. Merrill) is determined, in part, by the concentration of sucrose in the seed. Water stress during seed filling reduces photosynthesis, but the effect on seed sucrose concentration is not well understood. Soybean plants (cultivars Elgin 87 and Asgrow 232HS) were exposed to water stress from early in the seed-filling period until maturity in field and greenhouse experiments. Stress reduced yield (5–38%) and seed size (11–35%) but it did not affect the number of seeds. Stress accelerated leaf senescence as shown by a more rapid decline in leaf photosynthesis in both experiments. Seed-growth rate and cotyledon sucrose concentrations (expressed on a mM basis using the bulk water content of the cotyledons) during the linear phase of seed growth were not affected by stress in either experiment. Thus, water stress during seed filling had no direct effect on seed growth; its only effect was to accelerate leaf senescence, which shortened the seed-filling period and resulted in smaller seeds. Strategies to reduce yield loss from water stress during seed filling should focus on leaf senescence instead of seed growth.