In order to enhance the bioactivity of soy beverages, fermentation of soymilk was assayed using Lactobacillus rhamnosus CRL981 as a β-glucosidase producer. The biological activity of the fermented soy beverage enriched in isoflavone aglycones was determined by several methods to evaluate different antioxidant mechanisms. Fermented soymilk exhibited higher antioxidant activity (71.2 ± 4.0%) determined by β-carotene bleaching method but lower, although significant, DPPH radical scavenging effect (29.5 ± 0.9%) than the unfermented soymilk used as control. The fermented soymilk extracts were able to inhibit the oxidation of DNA induced by Fenton’s reagent. The increase of isoflavone aglycone contents during fermentation was a result of β-glucosidase activity towards isoflavone glucosides. This process allowed obtaining a soy beverage with enhanced antioxidant capacity able to contribute to the health and nutritional status improvement of consumers.