Background Due to the increasing use of informatics as an infrastructure in developing the quality of care and patient safety, informatics competency has become a crucial requirement for nurses. Aim This study was conducted to assess nursing informatics competency, and identify related factors in registered nurses. Method This research is an analytical descriptive study in which the research community included 205 nurses working at Tabriz University of Medical Sciences’ hospitals. The researcher-made questionnaire was developed as a data collection tool (α = 98%). The data was entered into SPSS16 software and correlation analysis and regression were carried out. Results The mean score percentage of total nursing informatics competency was 59.92%. The highest mean score was related to informatics skills (62.98), followed by the informatics knowledge subscale (59%). In addition, informatics competency was positively correlated with self-efficacy ( r = 0.27, p = 0.001), evidence-based practice ( r = 0.55, p = 0.001) and time spent on hospital information systems ( r = 0.16, p = 0.01). Conclusion Computer skills, self-efficacy, evidence-based practice and time spent on hospital information systems are determinant factors of nurses’ informatics competency. Developing nurses’ basic computer skills and incorporating informatics education programmes into the curriculum can enhance nurses’ informatics competency.