Hyponatremia is associated with poor prognosis in left heart failure and liver disease. Its prognostic role in pulmonary arterial hypertension (PAH) is not well defined. We investigated the association between hyponatremia and one-year mortality in two large cohorts of PAH. This study is a secondary analysis evaluating the association between hyponatremia and one-year mortality in patients treated with subcutaneous treprostinil (cohort 1). The results are validated using a PAH registry at a tertiary referral center (cohort 2). Eight-hundred and twenty patients were enrolled in cohort 1 (mean age = 47 ± 14 years) and 791 in cohort 2 (mean age = 55 ± 15 years). Sodium level is negatively correlated with mean right atrial pressure (r = −0.09, P = 0.018; r = −0.089, P = 0.015 in cohorts 1 and 2, respectively). In unadjusted analyses of cohort 1, the sodium level (as a continuous variable) is associated with one-year mortality (hazard ratio = 0.94; P = 0.035). Hyponatremia loses its significance (as a continuous variable and when dichotomized at ≤ 137 mmol/L; P = 0.12) when adjusted for functional class (FC), which is identified as the variable whose presence turns the effect of sodium level into non-significant. Secondary analyses using a cut-off value of < 135 mmol/L showed similar results. These results are validated in cohort 2. Although the sample size for patients with sodium < 130 mmol/L is small (n = 31), severe hyponatremia is associated with higher overall mortality (47% versus 23%; P = 0.01), even when adjusting for age, FC, and baseline 6-min walk distance (P < 0.001). Although baseline hyponatremia is associated with one-year mortality, it loses its significance when adjusted for FC.