BACKGROUND: Pulmonary rehabilitation (PR) is a programme of exercise and education and the most effective treatment for the symptoms and disability associated with chronic obstructive pulmonary disease. However, the benefits of PR are limited by poor uptake and completion. This trial will determine whether using trained volunteer lay health workers, called "PR buddies," improves uptake and completion of PR and is cost-effective. This trial protocol outlines the methods for evaluating effectiveness, cost-effectiveness, and acceptability. METHODS: The IMPROVE trial is a pragmatic, open, cluster randomised controlled trial planned in 38 PR services across England and Wales. PR services will be randomised to either intervention arm-offering support from PR buddies to patients with chronic obstructive pulmonary disease-or to usual care as the control arm. PR staff in trial sites randomised to the intervention arm will receive training in recruiting and training PR buddies. They will deliver training to volunteers, recruited from among people who have recently completed PR in their service. The 3-day PR-buddy training programme covers communication skills, confidentiality, boundaries of the PR-buddy role and behaviour change techniques to help patients overcome obstacles to attending PR. An internal pilot will test the implementation of the trial in eight sites (four intervention sites and four in control arm). The primary outcome of the trial is the uptake and completion of PR. A process evaluation will investigate the acceptability of the intervention to patients, PR staff and the volunteer PR buddies, and intervention fidelity. We will also conduct a cost-effectiveness analysis. DISCUSSION: Improving outcomes for chronic obstructive pulmonary disease and access to PR are priorities for the UK National Health Service (NHS) in its long-term plan. The trial hypothesis is that volunteer PR buddies, who are recruited and trained by local PR teams, are an effective and cost-effective way to improve the uptake and completion rates of PR. The trial is pragmatic, since it will test whether the intervention can be incorporated into NHS PR services. Information obtained in this trial may be used to influence policy on the use of PR buddies in PR and other similar services in the NHS. TRIAL REGISTRATION: ISRCTN12658458. Registered on 23/01/2023.