The increasing demand for sustainable agricultural practices has renewed interest in insect farming by-products such as insect frass. This study evaluated the potential of frass derived from Hermetia illucens larvae to suppress Fusarium wilt on tomato and to promote plant growth. Greenhouse experiments were conducted on potted tomato plants using heat-treated and untreated frass applied at different dosages, followed by a field trial under commercial farming conditions. Disease incidence, disease severity, plant biomass, yield, and crop development were assessed. Results showed a significant reduction of Fusarium wilt and an improvement of plant growth and yield, particularly at 10–20% frass application. However, double application in nursery and field did not further enhance plant performance, suggesting possible phytotoxic effects. These findings highlight the potential of insect frass as a sustainable alternative to mineral fertilizers and fungicides in tomato production.