Introduction <p>Competency in patient education is one of the main elements of nurses' professional competency. This study aimed to develop and evaluate the psychometric properties of a self-assessment scale regarding nurses’ competency in patient education.</p> Material and methods <p>This sequential exploratory mixed-methods study was conducted in two stages. The first stage explored nurses’ experiences and perceptions of competency in patient education. So, a qualitative conventional content analysis study was conducted in 2023–25 for item generation. Purposive sampling was used to recruit the participants. Competency in patient education was explained using three focused groups (<i>n</i> = 19), individual semi-structured interviews (<i>n</i> = 6), a written narrative, and a literature review.</p> <p>In the subsequent phase, the scale's face validity was evaluated with the involvement of 15 nurses, while the content validity ratio and content validity index were determined by eight and ten experts, respectively. Then, the exploratory factor analysis was used to determine the domains of the scale. 408 nurses were recruited by convenience sampling from all over Iran. The reliability of the scale was assessed using internal consistency, relative and absolute stability methods. As well, responsiveness, reproducibility, feasibility, and floor and ceiling effects were assessed.</p> Results <p>The scale was developed with 33 items and four dimensions: “adherence to the patient education principles”, “attending to patient preparation and conditions”, “observing ethical and interpersonal principles”, and “professionalism in patient education”, with a total variance of 57.44%. The findings confirmed that this tool is acceptable regarding validity, reliability, and other measurement features. The S-CVI/Ave of the scale was 0.9. The reliability of the scale was confirmed with Cronbach’s alpha coefficient (α = 0.917), (intra-cluster correlation = 0.977, with 95% confidence interval = 0.953–0.989), and (standard error of measurement = 2.15). The scale’s responsiveness, reproducibility, feasibility, and floor and ceiling effects were confirmed.</p> Conclusion <p>This scale was developed to assess nurses' competency in patient education.</p>