One in five people with COPD also lives with frailty. People living with both COPD and frailty are at increased risk of poorer health and outcomes, and face challenges to completing pulmonary rehabilitation. Integrated approaches that are adapted to the additional context of frailty are required. The aim of the present study is to determine the feasibility of conducting a randomised controlled trial of an integrated Comprehensive Geriatric Assessment for people with COPD and frailty starting pulmonary rehabilitation. This is a multicentre, mixed-methods, assessor-blinded, randomised, parallel group, controlled feasibility trial ("Breathe Plus"; ISRCTN13051922). We aim to recruit 60 people aged ≥50 with both COPD and frailty referred for pulmonary rehabilitation. Participants will be randomised 1:1 to receive usual pulmonary rehabilitation, or pulmonary rehabilitation with an additional Comprehensive Geriatric Assessment. Outcomes (physical, psycho-social and service use) will be measured at baseline, 90 days and 180 days. We will also collect service and trial process data, and conduct qualitative interviews with a sub-group of participants and staff. We will undertake descriptive analysis of quantitative feasibility outcomes (recruitment, retention, missing data, blinding, contamination, fidelity), and framework analysis of qualitative feasibility outcomes (intervention acceptability and theory, outcome acceptability). Recommendations on progression to a full trial will comprise integration of quantitative and qualitative data, with input from relevant stakeholders. This study has been approved by a UK Research Ethics Committee (ref.: 19/LO/1402). This protocol describes the first study testing the feasibility of integrating a Comprehensive Geriatric Assessment alongside pulmonary rehabilitation, and testing this intervention within a mixed-methods randomised controlled trial.