The response surface methodology was used to optimize the effect of three parameters, Bacillus subtilis (natto) Takahashi inoculum log (5–7 log CFU/g HDRB), initial water content (25%–45% w/v), and fermentation time (24–72 h), as independent variables on the extraction of the maximum water‐soluble proteins and peptides (WSPP) from heat‐stabilized defatted rice bran (HDRB). The optimum conditions were achieved by fitting the second‐order polynomial equation. The result showed that all the factors, initial water content, inoculum log, and fermentation time, were critical in extracting WSPP from HDRB. The predicted optimum conditions for highest amount of WSPP extracted were 40.96% w/v water content, 6.27 log CFU/g HDRB inoculum log, and 61.01 h fermentation time. Under these optimized conditions of solid‐state fermentation (SSF), 64.6% ± 0.7% of total protein in HDRB was extracted as WSPP which was closer to the predicted value of 63.3%. While the WSPP extracted from nonfermented HDRB was 20.6%. The majority of the WSPP from SSF‐HDRB had MW <5 kDa. The WSPP from SSF‐HDRB showed significant radical scavenging activities (56%, 44.4%, and 84.5%) in comparison to that from HDRB (12.5%, 9.3%, and 35.5%), of free radical (DPPH), superoxide radicals, and hydroxyl radicals, respectively. The SSF method is an efficient method to extract proteins and hydrolysates from HDRB and can find application as an ingredient in suitable products. Furthermore, the higher antioxidant activity is indicative of its potential function in controlling the oxidative stability in food products.