IntroductionIn Denmark, multiple national initiatives have been associated with improved bystander defibrillation and survival following out-of-hospital cardiac arrest (OHCA) in public places. However, OHCAs in residential neighbourhoods continue to have poor outcomes. The Cardiac Arrest in Residential Areas with MoBile volunteer responder Activation trial aims to improve bystander defibrillation and survival following OHCA in residential neighbourhoods with a high risk of OHCA. The intervention consists of: (1) strategically deployed automated external defibrillators accessible at all hours, (2) cardiopulmonary resuscitation (CPR) training of residents and (3) recruitment of residents for a volunteer responder programme.Methods and analysisThis is a prospective, pair-matched, cluster-randomised, superiority trial with clusters of 26 residential neighbourhoods, testing the effectiveness of the intervention in a real-world setting. The areas are randomised for intervention or control. Intervention and control areas will receive the standard OHCA emergency response, including volunteer responder activation. However, targeted automated external defibrillator deployment, CPR training and volunteer responder recruitment will only be provided in the intervention areas. The primary outcome is bystander defibrillation, and the secondary outcome is 30-day survival. Data on patients who had an OHCA will be collected through the Danish Cardiac Arrest Registry.Ethics and disseminationApproval to store OHCA data has been granted from the Legal Office, Capital Region of Denmark (j.nr: 2012-58-0004, VD-2018-28, I-Suite no: 6222, and P-2021-670). In Denmark, formal approval from the ethics committee is only obtainable when the study regards testing medicine or medical equipment on humans or using genome or diagnostic imagine as data source. The Ethics Committee of the Capital Region of Denmark has evaluated the trial and waived formal approval unnecessary (H-19037170). Results will be published in peer-reviewed papers and shared with funders, stakeholders, and housing organisations through summaries and presentations.Trial registration numberClinicalTrials.gov Registry (NCT04446585).