CONTEXT: The Dyspnea-12 (D-12) Questionnaire is a well-validated instrument in respiratory illnesses for breathlessness assessment, but its psychometric properties have not been tested in lung cancer. OBJECTIVE: To demonstrate the psychometric properties of the D-12 in lung cancer patients. METHODS: Baseline data from a lung cancer feasibility trial were adopted for this analysis. D-12 and a series of patient-reported tools, including five Numeric Rating Scales (NRS), the Hospital Anxiety and Depression Scale (HADS), and the Lung Cancer Symptom Scale (LCSS), were used for the psychometric assessment. Spearman's correlation coefficients (rs) were used to estimate the convergent validity of the D-12 with the NRS, HADS, and LCSS. Exploratory factor analysis was performed to examine construct validity. Reliability was tested by Cronbach's alpha and item-to-total correlations. D-12 score difference between patients with or without anxiety, depression, and chronic obstructive pulmonary disease (COPD) was explored to identify its discriminate performance. RESULTS: One hundred and one lung cancer patients were included. There were significantly positive correlations between the D-12 and the HADS, LCSS, and NRS measuring breathlessness severity and its associated affective distress. Factor analysis clearly identified two components (physical and emotional) of the D-12. Cronbach's alpha for D-12 total, physical, and emotional subscales was 0.95, 0.92, and 0.94, respectively. Patients with anxiety or depression demonstrated significantly higher D-12 scores than those without it, and patients with COPD reported significantly more severe breathlessness than those without COPD. CONCLUSION: The D-12 is a valid and reliable self-reported questionnaire for use in breathlessness assessment in lung cancer patients.