Duckweed (Lemna minor) has emerged as a promising alternative protein source in fish diets. The aim of this study was to evaluate the nutritional effects of Lemna minor as an alternative feed for juvenile black pacu (Colossoma macropomum) during the rearing stage. A completely randomized design with a fixed factor and four treatments was employed, using 204 juveniles distributed across 12 aquariums. Four different treatments were applied: T1 consisted of 100% Lemna minor meal; T2 was a formulation of fishmeal (46.2%), soybean (15.4%), and cassava (38.4%); T3 included fishmeal (37.2%), soybean (22.3%), cassava (16.9%), and Lemna minor (23.6%); and T4 used the commercial feed ITALCOL \({\circledR }\) Mojarra 32 (100%). Each treatment was applied in triplicate, administered twice daily, with biometric measurements taken weekly over 58 days, evaluating weight gain, feed conversion ratio (FCR), and final survival. The nutritional composition analysis revealed that Lemna minor meal alone did not meet the protein requirements (14.4%) necessary for juvenile black pacu during the rearing stage, although digestibility improved when combined with animal and oil-seed ingredients. Treatments T2 (26.11% protein) and T3 (29.92% protein) showed protein insufficiency, affecting growth and weight gain. However, T3, which combined fishmeal, soybean, cassava, and Lemna minor, achieved the best results in terms of weight gain, FCR (6.44%), and daily weight gain, comparing favorably with the commercial feed (T4). The study concluded that Lemna minor holds great potential as a partial replacement for conventional raw materials in fish diets.