In this study, we propose a squid jeotgal, Korean fermented seafood, supplement with different soy isoflavones supplements, followed by fermentation for different time intervals at 4°C to increase the antioxidant activity and improve the food value. In the first month, fermented jeotgals with at low concentration (2 mg g?1) of added soy isoflavones showed a significant increase in the activity of up to 55%, whereas, at high concentration (10 mg g?1), the activity almost doubled compared to that of the sample without isoflavones. Moreover, the squid enriched with isoflavones also exhibited significantly decreased total volatile base nitrogen, thiobarbituric acid reactive substances, and biogenic amines, indicative of higher inhibition of the formation of these substances. The changes in the microbial profile were also evaluated. This use of soy isoflavanones as an additive could aid in improving the nutritional value of fermented seafood to reduce the incidence of age-related and chronic disorders.