A wide variety of animal protein-based ingredients are commonly used in the pet food products. The raw ingredients and processing procedures used may greatly affect protein quality and digestibility. Testing the quality of alternative protein sources is necessary and contributes to the sustainability of pet foods. The objective of this study was to determine the chemical composition, nutrient digestibility, and nitrogen-corrected true metabolizable energy (TMEn) of 4 protein sources intended for use in dog and cat foods (pork peptone, calamari meal, chicken meal, and duck meal) using the precision-fed cecectomized rooster assay. Calamari meal and pork peptone had lower ash [4.4% and 3.6% of dry matter (DM), respectively], but greater crude protein (CP; 88.1% and 80.5% of DM, respectively) and gross energy (GE; 5.55 and 5.32 kcal/g of DM, respectively) compared with chicken meal (11.8% ash; 65.9% CP; 4.79 kcal/g) and duck meal (17.3% ash; 60.8% CP; 4.62 kcal/g). Acid-hydrolyzed fat (AHF) was lower in calamari meal (8.7% of DM) compared with the other proteins tested (15.5–15.9% of DM). Nutrient digestibility was variable among the protein sources [64 to 79% of DM, 76 to 83% of organic matter (OM), 86 to 92% of AHF, 83 to 89% of GE] with pork peptone having the highest DM, AHF, and GE digestibility, and calamari meal having the highest OM digestibility. Essential amino acid (AA) true digestibility was highest for calamari meal, with all AA having a digestibility greater than 90%. Except for histidine, all essential AA had a digestibility over 85% for pork peptone. All essential AA had a digestibility over 80% for duck meal, but chicken meal had 3 essential AA with digestibilities less than 80%. The TMEn of calamari meal (4.82 kcal/g DM; 86.8% of GE) was greater (P > 0.05) than that of pork peptone (3.83 kcal/g DM; 71.9% of GE), chicken meal (3.46 kcal/g DM; 72.2% of GE), and duck meal (3.46 kcal/g DM; 74.9% of GE). This study demonstrates the considerable variability that exists not only in the chemical composition, but also the nutrient digestibility, among protein sources intended for use in dog and cat foods.