<p>The increasing demand for protein has led to the development of insect farming as a sustainable and circular solution. This generates large quantities of frass (insect excreta), which has potential as an organic fertilizer. However, its effects on the soil–plant system remain underexplored. This study investigates the potential of black soldier fly (BSF) (<i>Hermetia illucens</i> L.) frass as an alternative fertilizer by assessing its impact on ryegrass (<i>Lolium multiflorum</i>) growth, nutrient uptake, and soil biota in comparison to mineral NPK fertilizer and poultry litter. A greenhouse pot experiment was conducted with four treatments: control, poultry litter and BSF frass applied at 5 t ha⁻¹, and NPK fertilizer applied at an equivalent nutrient level to frass. Plant biomass production, nutrient uptake, microbial activity, and earthworm biomass were measured across two harvests. At the first plant harvest, biomass production was similar across frass, NPK, and poultry litter treatments. By the second harvest, NPK fertilizer produced the highest biomass, followed by poultry litter and frass. Nitrogen uptake was significantly higher in the frass treatment during the second harvest (1.5 times higher than in NPK treatment), though phosphorus and potassium uptake were lower, likely limiting biomass production. Frass-treated soils exhibited the highest microbial activity, likely due to the presence of chitin stimulating chitinolytic microbial communities. Additionally, earthworm biomass was significantly higher in frass-treated soils, possibly due to increased microbial activity and organic matter. Thus,&#xa0;BSF frass can&#xa0;enhance soil health by stimulating microbial and earthworm activity&#xa0;but supplemental nutrients may be required&#xa0;to optimize plant growth, especially for nutrient-demanding crops. This study highlights the potential of frass as an organic fertilizer while identifying limitations that should be addressed for its effective agricultural application.</p>