Global population growth and the concomitant increase in biodegradable waste demand sustainable solutions for food production and waste management. Black soldier fly larvae (BSFL, Hermetia illucens) represent a promising approach by converting organic waste into protein-rich biomass and frass, a nutrient- and organic-matter-rich by-product. Frass contains nitrogen, phosphorus, potassium, micronutrients, chitin, and a beneficial microflora, and exhibits favorable physicochemical properties (pH 7-8, temperature 24-27 °C, low heavy metal content). Its nutritional composition varies according to the rearing substrate, thereby influencing the concentrations of N, P, K, and other mineral elements. BSFL frass has demonstrated fertilizing potential comparable to that of traditional organic amendments and can be used to stimulate plant growth, improve soil fertility, and enhance plant defense mechanisms through chitin and antimicrobial peptides. In aquaculture systems, frass promotes plankton production, including Chlorophyta and Chrysophyta, which serve as nutritional sources for fish larvae. However, application rates must be carefully optimized to avoid phytotoxicity or microbial imbalance. Overall, BSFL frass represents a multifunctional, environmentally friendly, and cost-effective organic resource that can be integrated into circular production chains, contributing simultaneously to waste valorization, reduced fertilizer costs, and the sustainability of agricultural and aquaculture systems.