The increasing demand for sustainable feed ingredients in aquaculture has driven research into alternative protein sources to replace fishmeal. This study evaluated the nutritional, physiological, and microbiological impacts of housefly (Musca domestica) larvae meal as a replacement for fishmeal in the diets of rainbow trout (Oncorhynchus mykiss). Diets were formulated to replace fishmeal with larvae meal at 50% and 100% inclusion levels. Growth performance, body composition, intestinal health, and gut microbiota were assessed over a 16-week feeding trial. Fish fed larvae meal-based diets exhibited comparable growth metrics, muscle yield, and hepatosomatic index to those fed fishmeal and commercial diets, indicating no compromise in growth efficiency. Intestinal histological examination revealed no diet-induced morphological changes, with all parameters remaining within normal ranges, highlighting the compatibility of larvae meal with fish intestinal health. Analysis of gut microbiota showed a shift toward Firmicutes dominance and increased abundance of Clostridiales in larvae meal-fed fish, suggesting enhanced fermentation activity. Blood chemistry parameters were consistent across dietary groups, supporting the physiological suitability of larvae meal-based diets. These findings demonstrate that housefly larvae meal is a sustainable and nutritionally viable alternative to fishmeal in aquafeeds for rainbow trout. The results underscore its potential to reduce reliance on fishmeal while maintaining fish health and growth performance.