Effects of water stress and foliar boron (FB) application on soybean (Glycine max (L) Merr.) seed composition and nitrogen metabolism have not been well investigated. Therefore, the objective of this study was to investigate the effects of water stress and FB on seed protein, oil, fatty acids, nitrate reductase activity (NRA), and nitrogenase activity (NA). A repeated greenhouse experiment was conducted where one set of soybean plants were subjected to water stress (WS), and the other set was watered (W). Foliar boron (B) was applied at rate of 0.45 kg·ha<sup>-1</sup>. Treatments were watered-plants with no FB (W), watered-plants with FB (WB), water-stress plants with no FB (WS), and water-stress plants with FB (WSB). The results showed that seed protein and oil percentage were significantly (P < 0.05) higher in WB than other treatments. Oleic acid increased and linolenic acid decreased in WB and WSB. Significant (P < 0.05) increase in NRA in leaves and roots and NA occurred in WB compared to W. In WSB, NRA in leaves and roots or nitrogenase activities were higher than those in WS. Nitrate reductase activity in nodules was greater in WB than in W, and was higher in WSB than in WS. The concentration of B in leaves and seed were significantly (P < 0.05) higher in W than in WS. Seed <sup>15</sup>N/ <sup>14</sup>N and <sup>13</sup>C/<sup>12</sup>C natural abundance were altered between watered-and watered-stressed plants. These results suggest that water stress and FB can influence seed composition, and nitrogen metabolism, and <sup>15</sup>N/<sup>14</sup>N and <sup>13</sup>C/<sup>12</sup>C ratios, reflecting environmental and metabolic changes in carbon and nitrogen fixation pathways. Lack of B translocation from leaves to seed under water stress may suggest a possible mechanism of limited B translocation under water stress. These findings may be beneficial to breeders to select for B translocation efficiency under drought conditions. Altered <sup>15</sup>N/<sup>14</sup>N and <sup>13</sup>C/<sup>12</sup>C under water stress can be used as a tool to select for drought tolerance using N and C isotopes in the breeding programs.
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