Introduction Today, 50% of people with cardiac disease do not participate in rehabilitation. The HeRTAstudy aims to develop and test a sustainable rehabilitation model supporting vulnerable patients in participating in rehabilitation and long-term physical activity. Methods A feasibility study with a non-blinded pilot randomized trial was developed in collaboration with partners and cardiac patients to test a multi-component rehabilitation intervention across hospital, municipality, and civil society. The study runs from January 2020 to December 2024. Eligibility criteria for participants: a) diagnosed with either ischemic heart disease, persistent atrial fibrillation, heart failure, or have had cardiac valve surgery, b) residents in Hvidovre Hospitals uptake area, c) cognitively functional, d) physically able to participate in rehabilitation. Patient recruitment will be located at Hvidovre Hospital, Capital Region of Denmark, data collection at Hvidovre Hospital, Rehabilitation Center Albertslund, the Danish Heart Association, and in two municipalities (Hvidovre and Brøndby). Patients in the control group have access to usual care at the hospital: rehabilitation-needs-assessment, patient education, and physical training. After or instead of hospital rehabilitation, the patient can be referred to municipal rehabilitation with patient education, and a total of 12 weeks of physical training across sectors. Patients in the intervention group will in addition to usual care, have access to an information book about cardiac disease, patient supporters from the Danish Heart Association, Information materials to inform employers about the employees' rehabilitation participation, a rehabilitation goal setting plan, a support café for relatives, and follow-up phone calls from physiotherapists 1 and 3 months after rehabilitation to support physical activities. Patients with vulnerabilities will additionally receive patient education conducted in small groups, pro-active counselling by a cardiac nurse, psychologist, or social worker, paid transportation to rehabilitation, and paid membership in a sports association. Patients are computer block-randomized so patients with vulnerability are distributed evenly in the two study arms by stratifying on a) a cut-off score of ≥ 5 in the Tilburg Frailty Indicator questionnaire and/or b) need of language translator support. A power calculation, based on an estimated 20% difference in participation proportion between groups, 80% power, a type 1 error of 5% (two-tailed), results in 91 participants in each study arm. The primary outcome: rehabilitation participation (attending ≥ two activities: patient education, smoking cessation, dietary counseling, and physical training) and reaching at least 50% attendance. Secondary outcomes: health-related quality of life, coping strategies, level of physical activities, and sustainability regarding participation in active communities after rehabilitation. The study is registered at ClinicalTrials.gov (NCT05104658). Results Differences between changes in outcomes between groups will be analyzed according to the intention-to-treat principle. Sensitivity analysis and analysis of the effect of the combined activities will be made. A process evaluation will clarify the implementation of the model, the partnership, and patients' experiences. Conclusion Cross-sectoral collaborations between hospitals, municipalities, and organizations in civil society may lead to sustainable and affordable long-term physical activities for persons with chronic illness. The results can lead to improve cross-sectoral collaborations in other locations and patient groups.