Background: Nocturnal hypoxemia is important in precapillary pulmonary hypertension (pPH) as it worsens pulmonary hemodynamics. Whether daytime oxygen saturation (SpO(2)) predicts nocturnal hypoxemia in pPH patients has not been conclusively studied. Objectives: To investigate the prevalence of nocturnal hypoxemia in comparison to daytime SpO(2) and disease severity in ambulatory patients with pulmonary hypertension. Methods: Consecutive patients diagnosed with pPH classified as either pulmonary arterial (PAH) or chronic thromboembolic pPH (CTEPH) had daytime resting and exercise SpO(2) (at the end of a 6-min walk test); thereafter, they underwent overnight pulse oximetry at home. Functional class, pro-brain natriuretic peptide (pro-BNP) and the tricuspid pressure gradient were assessed. Results: Sixty-three patients [median (quartiles) age 62 (53; 71), 43 females] with PAH (n = 44) and CTEPH (n = 19) were included. The resting SpO(2), exercise SpO(2), and mean nocturnal SpO(2) were 95% (92; 96), 88% (81; 95), and 89% (85; 92), respectively. Forty-nine patients (77%) spent >10% of the night with SpO(2) <90% (desaturators), and 33 (52%) spent >50% of the night with SpO(2) <90% (sustained desaturators). The positive predictive values of daytime SpO(2) >90% for being a nocturnal nondesaturator or sustained nondesaturator were 25 and 53%, respectively. Nocturnal SpO(2) was negatively correlated with the tricuspid pressure gradient, but not with functional class, 6-min walk test, or pro-BNP. Conclusions: Nocturnal hypoxemia is very common in PAH and CTEPH despite often normal daytime SpO(2) and reflects disease severity. Nocturnal pulse oximetry should be considered in the routine evaluation of pPH patients and research should be directed toward the treatment of nocturnal desaturation in pPH.