Introduction Patients with cardiac disease often experience anxiety (prevalence about 20%-25%) and have a doubled mortality risk when suffering from anxiety compared with patients without anxiety. This calls for interventions aiming to reduce anxiety. Methods and analysis The Heart Mind Trial consists of three parts: (1) screening of all hospitalised and outpatient cardiac patients with arrhythmia, heart failure or ischaemic heart disease at four university hospitals in Denmark using the Hospital Anxiety and Depression Scale-Anxiety subscale (HADS-A); Patients scoring ≥8 is invited to participate; (2) Assessment of the type of anxiety by Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders and (3) Randomised clinical superiority trial with blinded outcome assessment, with 1:1 randomisation to cognitive-behavioural therapy (CBT) performed by a CBT-trained cardiac nurse plus usual care or, usual care alone. The primary outcome is anxiety measured with HADS-A at 5 months. Secondary outcomes include anxiety symptoms measured with Becks Anxiety Inventory and heart rate variability. Exploratory outcomes measured at 12 months include blood cortisol (stress response), blood C reactive protein (stress response), health-related quality of life, readmission, mortality and attributable direct costs. A total of 336 patients will be included. The primary analyses are based on the intention-to-treat principle. For the primary outcome, we will use a linear regression model. For the long-term outcomes, mixed regression models will be used including repeated measurements. Ethics and dissemination The trial is performed in accordance with the Declaration of Helsinki. All patients must give informed consent prior to participation and the trial is initiated after approval by the Danish Data Protection Agency (P-2020-894) and the National Committee on Health Research Ethics (H-20066739). Positive, neutral and negative results of the trial will be published. Trial registration number NCT04582734.