Aims: Identify and synthesize evidence of nurses’ competency in electrocardiogram interpretation in acute care settings. Design: Systematic mixed studies review. Data sources: Cumulative Index to Nursing and Allied Health Literature, Medline, Scopus and Cochrane were searched in April 2021. Review methods: Data were selected using the updated Preferred Reporting Items for Systematic Reviews and Meta‑Analysis framework. A data-based convergent synthesis design using qualitative content analysis was adopted. Quality appraisal was undertaken using validated tools appropriate to study designs of the included papers. Results: Forty-three papers were included in this review. Skills and attitudes were not commonly assessed, as most studies referred to ‘competency’ in the context of nurses’ knowledge in electrocardiogram interpretation. Nurses’ knowledge levels in this important nursing role varied notably, which could be partly due to a range of assessment tools being used. Several factors were found to influence nurses’ competency in electrocardiogram interpretation across the included studies from individual, professional and organizational perspectives. Conclusion: The definition of ‘competency’ was inconsistent, and nurses’ competency in electrocardiogram interpretation varied from low to high. Nurses identified a lack of regular training and insufficient exposure in electrocardiogram interpretation. Hence, regular, standard training and education are recommended. Also, more research is needed to develop a standardized and comprehensive electrocardiogram interpretation tool, thereby allowing educators to safely assess nurses’ competency. Impact: This review addressed questions related to nurses’ competency in electrocardiogram interpretation. The findings highlight varying competency levels and assessment methods. Nurses reported a lack of knowledge and confidence in interpreting electrocardiograms. There is an urgent need to explore opportunities to promote and maintain nurses’ competency in electrocardiogram interpretation.