The sustainability limitations of fishmeal and fish oil have increased interest in nutrient- rich microalgal co-products for aquafeeds. This study evaluated the effects of processing on the nutritional composition and apparent digestibility of a defatted Nannochloropsis sp. QH25 co-product in rainbow trout (Oncorhynchus mykiss). Experiment 1 assessed the effects of extrusion at 90 °C or 127 °C, with or without precooking, and enzymatic treatment with protease, xylanase, and β-glucanase. Based on nutrient composition, the 90 °C extrusion treatment without precooking was selected for Experiment 2, which compared a reference diet with three diets containing 30% raw, extruded, or enzyme-treated co-product, using four replicate tanks per diet. Extrusion at 127 °C reduced crude protein, gross energy, and methionine concentrations, whereas extrusion at 90 °C largely preserved nutrient composition. Precooking reduced several essential amino acids but increased methionine and did not affect tryptophan. Crude protein digestibility was numerically higher in the extruded co-product than in the raw co-product, although the difference was not statistically significant. Most essential amino acids showed similar digestibility in the raw and extruded treatments, whereas enzymatic processing resulted in lower crude protein and amino acid digestibility. Omega-3 fatty acid digestibility remained high across diets. Overall, the raw co-product showed high nutrient digestibility, while extrusion resulted in only small, generally non-significant numerical differences and enzymatic processing generally resulted in lower nutrient digestibility under the conditions evaluated.