This study aimed to investigate the effects of dietary inclusion of Andean lupin (Lupinus mutabilis) on the growth performance, carcass composition, liver histology, and nutrient digestibility in rainbow trout (Oncorhynchus mykiss) juveniles. The experiment involved triplicate groups of fish (initial body weight 32.10 +/- 0.79 g) fed four extruded experimental diets with varying levels of Andean lupin (0, 50, 100, and 200 g kg(-1)) twice a day to apparent satiety over a period of 40 days. Faecal samples were collected daily for nutrient digestibility analysis. No significant correlations were observed between the dietary inclusion of Andean lupin and final weight, survival, weight gain, protein efficiency retention, thermal growth rate, feed conversion rate, and feed intake (P > 0.05). Similarly, there were no significant relationships between increasing dietary inclusion of Andean lupin and apparent digestibility coefficients for dry matter, crude protein, carbohydrate, and energy. However, the inclusion of Andean lupin led to a decrease in apparent digestibility coefficients for ash (R-2 =0.6, P < 0.05). Furthermore, the digestibility of Mg and Zn decreased in a quadratic manner (P < 0.01) with increasing levels of Andean lupin inclusion. The dietary treatments did not significantly affect whole-body composition, spleen-somatic index, and intestine-somatic index (P > 0.05). However, the hepatosomatic index decreased quadratically with increasing inclusion of Andean lupin (R-2 =0.51, P < 0.05), likely due to a slight reduction in cytoplasmic vacuolization of hepatocytes. In conclusion, this study demonstrates that Andean lupin is a promising plant protein ingredient for rainbow trout diets. It can be included at levels up to 200 g kg(-1) without adversely affecting growth performance and nutrient digestibility. These findings contribute to the expanding knowledge on alternative protein sources in aquaculture feed formulation.