Background: The complexities of the health care system have imposed complex ethical issues on nurses, resulting in moral distress in all categories of nurses with various specialties. Psychiatric nurses are also exposed to moral dilemmas because they care for vulnerable patients with delusions and suicidal ideation problems. Due to the different experiences of moral distress in different cultures, settings, and wards, we studied this phenomenon in Iranian psychiatric nurses. Objectives: We conducted this study to investigate the causes of moral distress in Iranian psychiatric nurses. Methods: This qualitative study was conducted based on a conventional content analysis on 12 psychiatric nurses selected by the purposive sampling method in 2020 in Shiraz, Iran, considering the maximum diversity. Data were collected through semi-structured interviews with participants for an average of 40 - 60 minutes per interview until data saturation. Results: We obtained causes of moral distress from 7 categories and 20 subcategories. The categories included “lack of professional competence”, “organizational culture”, “individual factors”, “environmental and organizational factors”, “management factors”, “weaknesses in professional and effective communication”, and “observation of moral dilemmas by nurses”. Conclusions: This study demonstrated various causes of moral distress that managers should consider by providing a safe environment and appropriate facilities for psychiatric nurses.