[Purpose] This study investigated functional capacity, physical activity, and respiratory and peripheral muscle strength in different functional classes of pulmonary arterial hypertension (PAH) compared with healthy subjects. [Subjects and Methods] This study included 31 patients with class II (n = 16) or class III (n = 15) PAH, classified according to the World Health Organization. Fifteen healthy subjects served as controls. Functional capacity was assessed using the 6-minute walk test (6MWT). Physical activity was determined using the International Physical Activity Questionnaire Short Form (IPAQ-SF). Respiratory muscle strength was measured using a mouth pressure device. Peripheral muscle strength was evaluated using a dynamometer. [Results] The 3 groups had similar demographic variables (p > 0.05). There were significant differences in 6MWT distance, maximal inspiratory pressure, and IPAQ categorical classification between the 3 groups (p < 0.05). Maximal expiratory pressure; total IPAQ score; and knee extensor, shoulder abductor, and hand grip strength were significantly lower in PAH patients (classes II and III) than in healthy subjects (p < 0.05). [Conclusion] As PAH progresses, respiratory muscle strength, functional exercise capacity, and physical activity decrease. Functional class should be taken into consideration when planning rehabilitation programs for this patient group.