Background Providing secondary prevention through structured and comprehensive cardiac rehabilitation programmes to patients after a myocardial infarction (MI) reduces mortality and morbidity and improves health-related quality of life. Cardiac rehabilitation has the highest recommendation in current guidelines. While treatment target attainment rates at Swedish cardiac rehabilitation centres is among the highest in Europe, there are considerable differences in service delivery and variations in patient-level outcomes between centres. In this trial, we aim to study whether centre-level guideline adherence and patient-level outcomes across Swedish cardiac rehabilitation centres can be improved through a) regular audit and feedback of cardiac rehabilitation structure and processes through a national quality registry and b) supporting cardiac rehabilitation centres in implementing guidelines on secondary prevention. Furthermore, we aim to evaluate the implementation process and costs.Methods The study is an open-label cluster-randomized effectiveness-implementation hybrid trial including all 78 cardiac rehabilitation centres (attending to approximately 10 000 MI patients/year) that report to the SWEDEHEART registry. The centres will be randomized 1:1:1 to three clusters: 1) reporting cardiac rehabilitation structure and process variables to SWEDEHEART every six months (audit intervention) and being offered implementation support to implement guidelines on secondary prevention (implementation support intervention); 2) audit intervention only; or 3) no intervention offered. Baseline cardiac rehabilitation structure and process variables will be collected. The primary outcome is an adherence score measuring centre-level adherence to secondary prevention guidelines. Secondary outcomes include patient-level secondary prevention risk factor goal attainment at one-year after MI and major adverse coronary outcomes for up to five-years post-MI. Implementation outcomes include barriers and facilitators to guideline adherence evaluated using semi-structured focus-group interviews and relevant questionnaires, as well as costs and cost-effectiveness assessed by a comparative health economic evaluation.Discussion Optimizing cardiac rehabilitation centres' delivery of services to meet standards set in guidelines may lead to improvement in cardiovascular risk factors, including lifestyle factors, and ultimately a decrease in morbidity and mortality after MI.Trial registration ClinicalTrials.gov. Identifier: NCT05889416. Registered 2023-03-23.