Introduction: Supervised exercise training (SET) is recommended as first-line therapy for symptomatic peripheral artery disease (PAD), but pain and limited tolerance often restrict the effective training dose. Eccentric exercise, such as downhill walking, elicits lower metabolic and cardiovascular responses and may represent a viable alternative to traditional walking. This case report describes the feasibility and physiological and functional effects of a 12-week downhill-walking SET program in a patient with symptomatic PAD. Methods: A 61-year-old man with symptomatic PAD completed a 12-week downhill-walking SET program performed at a preferred walking speed with progressively increasing negative slopes (−4% to −14%). Pre- and post-SET assessments included a graded treadmill test with near-infrared spectroscopy of calf muscle oxygenation, a 6-minute walk test, and lower-limb functional performance tests. Results: Following SET, pain-free (+6%) and maximal (+29%) walking distances improved. These changes were accompanied by reduced calf muscle oxygen desaturation during exercise, an enhanced calf muscle oxygenation overshoot at exercise onset, and faster postexercise reoxygenation. Lower-limb functional performance improved (20−25%), and the time to pain relief after walking tests was reduced (–46%), yet the 6-minute walking distance remained unchanged. Conclusion: This case report describes the implementation of a downhill-walking SET protocol in a patient with symptomatic PAD. The intervention was feasible and was accompanied by changes in walking performance, muscle oxygenation, and functional measures. These observations highlight the potential of an eccentric-based walking approach and support further investigation of this protocol in larger studies involving patients with PAD.