Pulmonary hypertension (PH) is a serious complication of chronic obstructive pulmonary disease (COPD), and there is no effective pharmacological treatment for COPD-associated PH. We evaluated the effect of udenafil, a phosphodiesterase-5 (PDE-5) inhibitor, on the exercise capacity of patients with severe COPD. Patients with severe and very severe COPD (forced expiratory volume in one second (FEV(1)) <50% of predicted) received udenafil (50 mg daily) for 8 weeks. A 6-min walk test (6MWT), lung function test, Doppler echocardiography, and Saint George's Respiratory Questionnaire (SGRQ) were completed before and after therapy. The primary outcome was a change in the 6-min walk distance (6MWD). Thirty-eight patients were screened for eligibility, and 23 completed the study. After 8 weeks of udenafil treatment, the mean 6MWD increased from 315 to 348 m (p = 0.02), and median PASP decreased from 36 to 30 mmHg (p = 0.02). There were no changes in the SGRQ score, Borg dyspnea score, or pulmonary function parameters. The PDE-5 inhibitor udenafil improved exercise capacity and decreased pulmonary artery pressure in patients with severe COPD. However, due to the small sample size, uncontrolled design and high dropout rate, the efficacy of udenafil in severe COPD needs to be confirmed in a large-scale randomized controlled study. This study was registered at ClinicalTrials.gov (number: NCT01364181).