INTRODUCTION: To enhance health and prevent secondary consequences for patients with cardiovascular disease (CVD), maintenance of an active lifestyle following participation in cardiac rehabilitation (CR) is important. However, levels of physical activity often decrease after completion of a structured CR programme. Models that support long-term behaviour change with a sustained level of physical activity are imperative. The aim of this study is to evaluate the feasibility of a mobile health intervention based on the Health Action Process Approach theoretical model of behaviour change in patients with CVD for 3 months after completion of a CR programme. METHODS AND ANALYSIS: In a feasibility trial design, we will recruit 40 participants from CR programmes at Slagelse Hospital, the City of Slagelse (municipality), or Holbæk Hospital. After completing the standard structured CR programme, each participant will create an action plan for physical activity together with a physiotherapist. Following that, participants are sent 2 weekly text messages for 3 months. The first text message prompts physical activity, and the second will check if the action plan has been followed. If requested by participants, a coordinator will call and guide the physical activities behaviour. The feasibility of this maintenance intervention is evaluated based on predefined progression criteria. Physical activity is measured with accelerometers at baseline and at 3 months follow-up. ETHICS AND DISSEMINATION: Study approval was waived (EMN-2021-00020) by the Research Ethics Committee of Region Zealand, Denmark. Study results will be made public and findings disseminated to patients, health professionals, decision-makers, researchers and the public. TRIAL REGISTRATION NUMBER: NCT05011994.