Breathlessness is a cardinal symptom of pulmonary arterial hypertension (PAH); yet no breathlessness instrument has been previously tested for reliability and validity for this population. Using a cross-sectional design, we tested the psychometric properties of the Dyspnoea-12 (D-12), for the assessment of breathlessness in PAH. Pearson’s correlations with World Health Organization functional class (WHO FC), Minnesota Living with Heart failure – pulmonary hypertension modified version (MLHF-PH), Hospital Anxiety and Depression scale (HADS) and 6-minute walk distance test (6MWD) were conducted. Participants (n = 176) were mostly female (70.5%), mean age 54.3±14 years; diagnosed with idiopathic PAH (48.9%), congenital heart disease (27.8%) and connective tissue disease (23.3%); and most were WHO FC II (32.4%) and III (52.3%). The D-12 showed excellent internal consistency for the total and two-component scores for physical and emotional (Cronbach’s α 0.95, 0.93 and 0.94, respectively). D-12 total score was significantly associated with MLHF-PH (r = 0.70), HADS (anxiety r = 0.54 and depression r = 0.68), WHO FC (r = 0.49), and 6MWD (r = -0.26). In patients with PAH, the D-12 - a short patient reported measure of breathlessness severity that taps the physical and emotional components, is a reliable and valid instrument.