Background-Elevated right atrial (RA) pressure is an established prognostic measure in pulmonary arterial hypertension (PAH). However, little is known about perturbations in RA function in PAH. Methods and Results-Reservoir (RA longitudinal strain [RA LS]), conduit (RA early LS rate), and active (RA late LS rate) phases were assessed by 2D speckle tracking in 65 patients with PAH, 6-minute walk distance ≤450 m, and a pulmonary vascular resistance >800 dynes·s/cm5, despite therapy with at least 2 PAH-specific medications enrolled in the Imatinib in Pulmonary Arterial Hypertension, a Randomized Efficacy Study (IMPRES) trial and were compared with 30 healthy controls of similar age and sex. We studied the association of RA functional measures with invasive pulmonary hemodynamics, cardiac structure and function, and N-terminal pro brain natriuretic peptide. RA LS and early LS rate were reduced in PAH patients compared with controls (27.1±11.6 versus 56.9±12.7, adjusted P<0.001 and-0.6±0.5 versus-1.5±0.5, adjusted P<0.001, respectively) even after adjusting for RA area and invasive RA pressure, whereas RA late LS rate was similar between groups (-1.4±0.7 versus-1.5±0.4, P=0.42). Among PAH patients, worse RA LS correlated with greater RA size (r=-0.50, P<0.0001) and pressure (r=-0.37, P=0.002), but not pulmonary artery pressure (r=-0.07, P=0.58). Worse RA LS was also associated with right ventricular enlargement and dysfunction and higher N-terminal pro brain natriuretic peptide independent of RA size and pressure. Conclusions-RA reservoir and passive conduit functions are impaired in PAH, independent of RA size and pressure, and likely reflect right ventricular failure and overload.