Immature seeds of soybean are becoming increasingly popular as a snack/vegetable to harness the health benefits of soybean. They are shelled from the immature pods picked from the mother plant at different reproductive stages. Information concerning changes in antioxidant constituents and antioxidant capacity during reproductive phases of soybean seeds is scarce. The aim of the present study was to determine whether immature seeds picked at different reproductive stages differ in tocopherol, isoflavone, total phenolic contents, free radical scavenging activity, and total antioxidative capacity. Seeds shelled from the soybean pods picked at three reproductive stages (R5, R6, and R7) as well as at full maturity were subjected to high-performance liquid chromatography analyses for tocopherol and isoflavone contents. Significantly higher values (P < 0.05) were observed for tocopherols and isoflavones in immature seeds picked at late reproductive stages. At the first reproductive stage, that is, R5 stage, δ-tocopherol was the predominant form of tocopherol, whereas in subsequent reproductive stages as well as at complete maturity stage, the γ-isomer contributed maximum proportion to the total tocopherol content. Genistein was, in general, the major form of isoflavone at all reproductive stages. Reduction in free radical scavenging activity, total antioxidant capacity, and total phenolic content in late-picked seeds concomitant with increased concentration of tocopherol and isoflavone isomers was observed. The results show that bioactive constituents other than isoflavones and tocopherols may decline with the advancement of maturity.