Liver dysfunction is an important determinant of the prognosis of left heart failure patients. However, few studies have reported on cardiohepatic interactions in right heart failure patients, a condition that is an important prognostic factor in pulmonary arterial hypertension (PAH). This study aimed to evaluate the existence and extent of hepatic fibrosis and its contribution as a prognostic factor in PAH. This prospective study enrolled 60 consecutive patients with PAH in the International University of Health and Welfare Mita Hospital from June 2016 to December 2017. After the application of the exclusion criteria, 35 patients were assessed for hepatic fibrosis, using real-time tissue elastography, and for clinical deterioration. Sixteen healthy controls were also assessed for comparison. The liver fibrosis index (LFI) was significantly higher in PAH patients than in healthy controls (1.214 ± 0.047 vs. 0.911 ± 0.059, P < 0.001), suggesting that PAH patients exhibited mild liver fibrosis. However, patients with deterioration (vs. no deterioration) had significantly higher LFI values (1.507 ± 0.078 vs. 1.080 ± 0.034, P < 0.001), independent from other established liver function parameters. A receiver operating characteristic curve analysis identified that an LFI ≥ 1.275 indicated a significant probability of clinical deterioration (hazard ratio: 8.4 (95% CI 1.5–45.4, P = 0.012), independent from other known PAH prognostic factors. PAH patients may exhibit subclinical liver fibrosis associated with chronic right heart failure. The LFI can serve as both a non-invasive evaluation of liver fibrosis and a predictive marker for the deterioration of PAH patients.