Pulmonary hypertension (PH) is characterized by a progressive remodelling of the pulmonary vasculature resulting in right heart failure and eventually death. The serotonin transporter (SERT) may be involved in the pathogenesis of PH in patients with chronic-obstructive pulmonary disease (COPD). This study investigated for the first time the SERT in vivo availability in the lungs of patients with COPD and PH (COPD+PH). SERT availability was assessed using SERT-selective [11C]DASB and positron emission tomography/computed tomography (PET/CT) with dynamic acquisition over 30 min in 4 groups of 5 participants each: COPD, COPD+PH, pulmonary arterial hypertension, and a healthy control (HC). Time activity curves were generated based on a volume of interest within the middle lobe. Tissue-to-blood concentration ratios after 25 to 30 min (TTBR25-30) served as receptor parameter for group comparison and were corrected for lung tissue attenuation. Participants underwent comprehensive pulmonary workup. Statistical analysis included group comparisons and correlation analysis. [11C]DASB uptake peak values did not differ among the cohorts after adjusting for lung tissue attenuation, suggesting equal radiotracer delivery. Both the COPD and COPD+PH cohort showed significantly lower TTBR25-30 values after correction for lung attenuation than HC. Attenuation corrected TTBR25-30 values were significantly higher in the COPD+PH cohort than those in the COPD cohort and higher in non-smokers than in smokers. They positively correlated with invasively measured severity of PH and inversely with airflow limitation and emphysema. Considering all COPD patients ± PH, they positively correlated with right heart strain (NT-proBNP). By applying [11C]DASB and PET/CT, semiquantitative measures of SERT availability are demonstrated in the lung vasculature of patients with COPD and/or PH. COPD patients who developed PH show increased pulmonary [11C]DASB uptake compared to COPD patients without PH indicating an implication of pulmonary SERT in the development of PH in COPD patients.