Fermentation technology is essential for raising feed's nutritional content and improving quality. The purpose of the current feeding study was to assess the impact of switching from raw rice bran to fermented rice bran (FRB) on the growth performance, nutritional consumption, economic value, and histological alterations in the gut and liver of the grey mullet, Mugil cephalus (with an initial weight of 5.7 ± 0.01g). The control diet (c) and the other three isoproteic (27% crude protein) diets (B50, B75, and B100) were formulated with FRB replacing 50, 75, and 100% of the rice bran, respectively. Following a 60-day feeding period, the grey mullet given the B100% diet had greater final weight, weight gain, specific growth rate, and protein contents in their bodies compared to the control group (P< 0.05). Additionally, the fish given the B100% diet showed improved feed conversion ratio, protein efficiency ratio, and feed efficiency ratio values. Furthermore, the insertion of FRB resulted in a substantial increase in the count of intestinal microbiota (P< 0.05). In contrast to the control group, the fermented therapies had no detrimental effects on the overall immunity. From an economic perspective, the cost of formulating the diet increased by 0.88% when rice bran was fully replaced with fermented rice bran (FRB) (B100). The results of this study suggest that substituting rice bran with FRB up to 100% may improve growth performance, feed consumption, gut health, and profitability in juvenile M. cephalus.