Introduction Psychiatric health represents one of the most complex fields for nursing due to the nature of the patients, who may express various forms of maladaptive behaviours or aggression. Additionally, the role of psychiatric nurses extends beyond their patients to their patients’ families and communities. Therefore, nurses must be highly aware of psychiatric patients’ needs and legal rights in accordance with established ethical and legislative standards. Psychiatric health nurses are the healthcare team members that spend the most time with their patients, which can expose nurses to more ethical problems, particularly when working with mentally disturbed patients. In Egypt, inclusive legislation for mental health has been enacted to safeguard the rights of mentally ill patients, which includes the Egyptian Nursing Profession Code of Ethics, established by the Egyptian Nursing Syndicate (Egyptian Nursing Syndicate, 1976), and the Egyptian New Mental Health Law 71/2009. However, many current psychiatric health nursing staff are not aware of these legislations and codes, which might lead to frequent violations of patient rights. This study was conducted as a first step toward improving psychiatric nurses’ knowledge of the legal and ethical issues associated with the treatment of psychiatric patients. Aim: To assess the effects of an educational intervention on psychiatric nurses’ knowledge regarding legal and ethical issues associated with psychiatric patient treatment. Methodology Design: A quasi-experimental, single-group, pre-test–post-test design was used. Setting: Al-Abbasia Mental Health Hospital in Cairo, Egypt. Sample: A convenience sample of 50 psychiatric nurses was recruited. Methods: Structured, self-administered knowledge questionnaire developed by the researchers was used to assess psychiatric nurses’ knowledge levels regarding legal and ethical responsibilities. Results: Nurses’ satisfactory knowledge before the intervention ranged between 4% for legal aspects to 62% for violence and neglect. All areas of knowledge significantly improved after the intervention, with satisfactory knowledge ranging between 76% for definitions to 98% for neglect and unprofessional practices. Overall, only one (2%) nurse had satisfactory total knowledge before the intervention, which increased to 90% after the intervention (p<0.001). Nurses’ knowledge before the intervention was significantly and positively correlated with their qualifications. After the intervention, knowledge correlated negatively with their age and years of experience. Conclusion: In light of the current study findings, educational interventions appear to be effective for improving psychiatric nurses’ knowledge of the legal and ethical issues related to their practice. Although only one (2%) nurse had a satisfactory total knowledge score before the intervention, this proportion rose to 90% after the intervention. Recommendations Based on the study findings, the following recommendations are suggested: this educational intervention could be used in similar settings, with further amelioration through the addition of a practical application section. The long-term effectiveness of the program must also be tested through a follow-up study, and the impacts of improved knowledge on the quality of patient care also require further research. This educational intervention is recommended to be included as a basic component of the orientation program for new staff and as part of in-service education programs for nursing staff working in similar settings.