Protein-energy malnutrition (PEM) is a persistent public health challenge in most developing countries. This study investigated the protein nutritional value of wheat and wheat-sorghum biscuits fortified with longhorn grasshopper (Ruspolia differens) to determine their suitability as a supplementary food. Fourteen diets were fed to male weanling Sprague-Dawley rats, including 11 isonitrogenous diets with 10% protein prepared from 10 biscuit variations and skimmed milk powder as a reference, 1 protein-free diet, and 2 rehabilitation diets made with wheat and wheat-sorghum biscuits fortified with 40% Ruspolia differens powder (RDP). The protein efficiency ratio, food efficiency ratio, true and apparent protein digestibility, and net protein retention ratio results for the fortified biscuit diets were all negative, with the exception of wheat biscuits supplemented with RDP 40%. The isonitrogenous diets maintained the rats with no substantial weight gain or loss. Rats on rehabilitation diets gained weight rapidly, by 61% and 69% for wheat-sorghum and wheat diets, respectively. Weight increase during rehabilitation was considerably higher (P < 0.05) by 50% and 54.6% in wheat-sorghum and wheat diet groups compared to normal growth phases. The rehabilitated rats had a greater percentage of body weight than the experimental groups. The protein digestibility-corrected amino acid score (PDCAAS) of fortified biscuits ranged from 26% to 33% for wheat biscuits and 22% to 32% for wheat-sorghum biscuits. The digestible indispensable amino acid score (DIAAS) of the fortified biscuits varied from 47% to 60% for wheat biscuits and 44% to 60% for wheat-sorghum biscuits. As a result, the fortified biscuits fell short of the minimum requirements of 70% for PDCAAS and 75% for DIAAS in fortified protein diets. Furthermore, it appears that substituting 10% RDP to wheat-sorghum and wheat biscuits does not promote rat growth. Fortification with 40% RDP, on the other hand, dramatically improves rat’s growth and rehabilitation. Because of their high protein nutritional content, the biscuits fortified with 40% RDP could be used as a supplementary to help rehabilitate malnourished children.