Introduction: In patients treated for cardiac disease, loneliness is known to contribute negatively to health behavior, health outcome, and increase risk of cardiac and all-cause mortality. Even so, in health care research, social support interventional studies targeting patients who experience loneliness are lacking. Aim: To determine the feasibility of an individually structured social support intervention targeting patients treated for cardiac disease who experience loneliness. Design: A feasibility study based on randomized clinical trial design with 1:1 randomization to a 6-month social support program, plus usual care (intervention) versus usual care, (i.e., regular guidelines-based follow-up). Intervention: Patients classified as high risk lonely according to the High Risk Loneliness tool will be provided with an informal caregiver in the 6 months rehabilitation phase following cardiac disease treatment. The informal caregiver will be designated by the patient from the existing social network or a peer, depending on patients’ preferences. The core content of the intervention is through nurse consultations at baseline and 1, 3, and 6 months, to enhance and reinforce the informal caregiver’s competences to be a social support resource. The theoretical framework of the nurse consultations will be based on middle-range theory of self-care. Outcome: Feasibility will be evaluated in terms of acceptability and adherence according to predefined feasibility criteria. The preliminary effect of the intervention on patient-reported outcomes, health behaviors, and health outcomes will be evaluated in the intervention and the control group at baseline and 1, 3, 6, and 12 months. Discussion: The present study will contribute with knowledge on how to implement a feasible social support intervention targeting patients treated for cardiac disease who experience loneliness and, furthermore, investigate the preliminary effect on health behavior and health outcome in the early rehabilitation period. Trial registration: The trial is registered on clinicaltrials.gov (NCT05503810) 18.08.2022.