Purpose <p>The growing population of aging dogs has stimulated a demand for functional dietary ingredients that can support long-term health and metabolic stability. Antioxidant properties and digestive stability of porcine liver hydrolysates produced by enzymatic hydrolysis and subsequent Maillard reaction were investigated as a potential functional ingredient in pet food.</p> Methods <p>Porcine liver was hydrolyzed using alcalase, followed by high-temperature treatment with or without xylose to induce Maillard reactions. Three treatment groups were prepared: porcine liver hydrolysate (PLH), heat-treated hydrolysate (PLH-H), and hydrolysate subjected to xylose-induced Maillard reaction (PLH-HX). Antioxidant activity was evaluated before and after simulated canine gastrointestinal digestion to assess functional stability relevant to pet food applications.</p> Results <p>The Maillard reaction substantially enhanced antioxidant activity prior to digestion, particularly in radical-scavenging assays. However, following in vitro digestion, antioxidant activity decreased in the Maillard reaction-treated samples, and differences between the treatments were reduced. Digestibility was also lower in the Maillard reaction-treated samples than in the non-treat samples. Although Maillard reaction improved the initial antioxidant capacity, its stability under gastrointestinal conditions was limited and assay-dependent.</p> Conclusion <p>The combined hydrolysis–Maillard reaction strategy modulated the structural and redox characteristics of porcine liver by-products without complex processing, demonstrating its potential for use as a natural antioxidant ingredient in companion animal feed. Further in vivo validation is required to confirm the physiological relevance of these findings.</p>