Abstract Body (Do not enter title and authors here): Background: Right ventricular failure is a major prognostic determinant of pulmonary hypertension and progresses with myocardial fibrosis. Extracellular matrix proteins play a major role in myocardial fibrosis. Right ventricular biomarkers for right ventricular function and fibrosis are poorly examined in pulmonary hypertension. This study investigated the significance of collagen triple helix repeat-containing protein 1 (CTHRC1), one of the extracellular matrix proteins, as a right ventricular biomarker. Methods and Results: A monocrotaline-induced pulmonary hypertension rat model was used in this study. CTHRC1 expression in the right ventricle was higher in rats with monocrotaline-induced pulmonary hypertension than in controls. CTHRC1 was associated with fibrosis in the rat's right ventricle. Next, human circulating CTHRC1 levels were evaluated in controls (n = 20), pulmonary arterial hypertension (PAH, n = 46), and chronic thromboembolic pulmonary hypertension (CTEPH) patients (n = 64). Human circulating CTHRC1 levels were higher in PAH (P = 0.006) and CTEPH patients (P = 0.011) than in controls (Figure). Human circulating CTHRC levels were correlated with tricuspid lateral annular peak systolic velocity (R = -0.213, P = 0.029), right ventricular fractional area change (R = -0.225, P = 0.017), and right ventricular global longitudinal strain evaluated by magnetic resonance imaging (R = 0.429, P = 0.009). In addition, human circulating CTHRC1 levels were decreased after balloon pulmonary angioplasty (P < 0.001) in CTEPH patients. Conclusions: CTHRC1 could be a novel biomarker associated with right ventricular failure and fibrosis in pulmonary hypertension patients.