IntroductionSimple and scalable strategies are needed to improve ‘out-of-hospital’ support and management for people living with cardiovascular disease (CVD) and respiratory disease. Text messaging via mobile phones has been shown to be effective in helping promote lifestyle change and is supported by quantitative and qualitative evidence. The aim of this study is to test the effectiveness and implementation of a 6-month text messaging support programme for people with CVD and respiratory disease as an addition to cardiac and pulmonary outpatient rehabilitation.Methods and analysisPragmatic randomised controlled trial (n=310) to test the effectiveness of a 6-month text message support programme on clinical outcomes in people with CVD and chronic respiratory disease who are attending outpatient cardiac and pulmonary rehabilitation. The study includes a nested process evaluation to inform scalability and implementation across settings. The intervention group will receive a text message support programme comprising five messages per week for 26 weeks and the control group will continue with standard care. The primary outcome is exercise capacity (6 min walk distance). Secondary outcomes include clinical measures (proportion of people meeting the Australian guideline-recommended blood pressure and cholesterol targets), lifestyle outcomes (smoking rates, achievement of national guidelines for nutrition and physical activity), quality of life, mood (Hospital Anxiety and Depression Scale), medication adherence and attendance at and completion of rehabilitation.Ethics and disseminationPrimary ethics approval was received from the Sydney Local Health District Hospital Human Research Ethics Committee and associated Governance committees at sites. Results will be disseminated via the usual scientific forums including peer-reviewed publications and presentations at international conferences. At its conclusion, the study will determine the effectiveness and implementation of a simple programme that aims to improve health outcomes and attendance at rehabilitation for people with CVD and chronic respiratory disease.Trial registration numberACTRN12616001167459