Fast-paced growth in global aquaculture has elevated concerns about the high costs of fish farming production and potential water pollution. Thus, finding eco-friendly and more sustainable alternative protein sources for fish diets is of vital importance to the industry. A twin-screw extrusion processing study was performed using three ingredient blends formulated with graded levels of high protein fermented soybean meal (FSBM) (0, 80% and 100% db fishmeal replacement) along with calculated amounts of other ingredients to meet the rainbow trout diets’ requirements. Increasing the FSBM content from 0% to 100% resulted in a substantial increase in brightness, greenness, and yellowness, and a decrease in bulk density, water absorption index, and unit density (UD) values of the extrudates by 12.5%, 73%, 30%, 7.3%, 27.5%, and 10%, respectively. Compared to the control diet (100% fishmeal-based), extrudate moisture contents increased by 15.2% and 22% for the diets containing 80 and 100% FSBM, respectively; although no change was observed by increasing FSBM from 80 to 100%. The highest water solubility index (WSI) was obtained for 80% FSBM; however, further increasing FSBM did not influence the WSI significantly. All extrudates exhibited low water activity and high pellet durability values (less than 0.5 and more than 99.5%, respectively). The most buoyant extrudates were obtained using total FSBM inclusion, with UD and expansion ratio values of nearly 660 kg/m3 and 1.3, respectively. Overall, results indicated that FSBM could be a promising alternative protein in vegetable-based blends.