This study aimed to investigate whether fermented rice bran can substitute soybean meal in the diet of the grey mullet. Four different substitution levels (0, 5, 10, and 15%) in the diet were investigated for soybean meal replacement with fermented rice bran (FRB). In the 0–10% replacement groups, weight, specific growth rate, feed efficiency ratio, body protein level, and protein efficiency ratio were significantly higher (P<0.05) compared to the 15% replacement group. Additionally, there was no difference (P>0.05) in the grey mullet's growth rate in the 0 and 10% replacement treatments. Furthermore, replacing up to 15% of soybean meal with fermented rice bran (FRB) did not significantly affect the overall immunity of the grey mullet. Additionally, incorporating fermented rice bran (FRB) significantly increased the gut microbiota counts (P< 0.05). The greatest increase in the gut microbiota was observed when up to 5% of soybean meal was replaced with FRB. From an economic perspective, substituting up to 5% of soybean meal with FRB reduced the cost of formulating the diet by 0.64% and decreased the cost per kilogram of weight gain by 5 to 10%, or approximately 9.9 and 1.5%, respectively. Overall, the study suggests that replacing up to 10% of rice bran with FRB can enhance growth performance, feed consumption, body composition, intestinal microbiota counts, and financial viability in the grey mullet (Mugil cephalus).