Introduction: Reduced functional capacity is a hallmark of pulmonary arterial hypertension (PAH). Six minute walk distance (6MWD) is a common measure of functional capacity. Unlike the 6MWD, daily activity monitoring integrates behavioral aspects of physical capacity. We hypothesized that daily activity is associated with 6MWD and quality of life (QOL) in patients with PAH. Methods: This study included patients with PAH from the NHLBI-funded Pulmonary Vascular Disease Phenomics (PVDOMICS) study. PVDOMICS collected 6MWD and QOL surveys (Minnesota Living with Heart Failure [MLHF] and Short Form-36 [SF-36]) data. Daily activity was measured using Fitbit devices which participants wore continuously for 12 weeks. Daily activity was expressed as average daily steps and graded intensity. Results: We enrolled 45 patients with PAH (age, 48.1 + 13.4 years) with median daily steps of 4146 (IQR: 2682-6977). Sedentary, fairly active, and very active minutes/day were 850 (IQR: 774-1093), 5.0 (IQR 2.0-7.6), and 2.1 (IQR 0.7-7.3). The median time between 6MWD and the start of daily activity monitoring was 32 days (IQR 11-53). Average daily steps correlated with 6MWD (r = 0.70, p<0.001), MLHF total score (r = - 0.46, p=0.002), MLHF physical and emotional domains, and the SF-36 physical component score. Among hemodynamic measures, average steps and fairly active minutes were most strongly associated with PA compliance (r=0.46 and r=0.64; both p<0.05). In contrast, 6MWD only correlated with the SF 36 physical component but no MLHF measures. Conclusions: In a geographically diverse PAH cohort, long-term daily step counts are associated with exercise capacity and QOL. Daily activity measures capture behavioral information not captured by the 6MWD and correlated with prognostic hemodynamic measures. Daily activity monitoring and 6MWD represent complementary measures for the assessment of functional capacity in patients with PAH.