INTRODUCTION: Physical inactivity is a risk factor for repeat cardiac events and all-cause mortality in coronary heart disease (CHD). Cardiac rehabilitation, a secondary prevention programme, aims to increase physical activity levels in this population from a reported low baseline. This trial will investigate the effectiveness and implementation of a very brief physical activity intervention, comparing different frequencies of physical activity measurement by cardiac rehabilitation clinicians. The Measure It! intervention (<5 min) includes a self-report and objective measure of physical activity (steps) plus very brief physical activity advice. METHODS AND ANALYSIS: This type 1 hybrid effectiveness-implementation study will use a two-arm multicentre assessor-blind randomised trial design. Insufficiently active (<150 min of moderate-to-vigorous physical activity per week) cardiac rehabilitation attendees with CHD (18+ years) will be recruited from five phase II cardiac rehabilitation centres (n=190). Patients will be randomised (1:1) to five physical activity measurements or two physical activity measurements in total over 24 weeks. The primary effectiveness outcome is accelerometer daily minutes of moderate-to-vigorous intensity physical activity at 24 weeks. Secondary effectiveness outcomes include body mass index, waist circumference and quality-of-life. An understanding of multilevel contextual factors that influence implementation, and antecedent outcomes to implementation of the intervention (eg, feasibility and acceptability), will be obtained using semistructured interviews and other data sources. Linear mixed-effects models will be used to analyse effectiveness outcomes. Qualitative data will be thematically analysed inductively and deductively using framework analysis, with the framework guided by the Consolidated Framework for Implementation Research and Theoretical Domains Framework. ETHICS AND DISSEMINATION: The study has ethical approval (University of Canberra (ID 11836), Calvary Bruce Public Hospital (ID 14-2022) and the Greater Western Area (ID 2022/ETH01381) Human Research Ethics Committees). Results will be disseminated in multiple formats for consumer, public and clinical audiences. TRIAL REGISTRATION NUMBER: ACTRN12622001187730p.