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The long-horned edible grasshopper Ruspolia differens (Serville) (Orthoptera: Tettigoniidae) is a significant food source in Southern and Eastern Africa due to its rich nutritional profile. The declining yields from wild harvests have driven research into developing methods for mass rearing of R. differens. Despite extensive efforts, identifying cost-effective nutritious diets has been challenging. This study tested eight diets including seven diets constituted from oil seed by-products, i.e., sunflower seed cake, shea seed cake, oil palm seed cake and one control diet (germinated finger millet). The experiment was arranged as a randomised unreplicated block design. Survival of neonates to adults and adult weight of R. differens reared on sole sunflower seed cake and combinations of either oil palm seed cake or shea seed cake with germinated finger millet were statistically comparable to the R. differens reared on sole germinated finger millet (control); whereas no R. differens survived on any of the diets containing shea seed cake and the sole oil palm seed cake diet. Germinated finger millet increased the insect’s crude fat, carbohydrates and mono-unsaturated fatty acids. Conversely, R. differens reared on oil seed by-products-based diets had the highest contents of protein, minerals and polyunsaturated fatty acids than those raised on germinated finger millet. We conclude that (i) sole sunflower seed cake and/or a combination of the same or oil palm seed cake with 50% germinated finger millet are alternative diets to sole germinated finger millet in the mass rearing of R. differens and (ii) the diets based on oil seed by-products can be used to enhance the content of protein, mineral and polyunsaturated fatty acids of R. differens. We recommend further studies to increase the content of alpha linolenic acid in these diets to produce R. differens which is more nutritionally suitable for human consumption.