Abstract Background Chronic obstructive pulmonary disease (COPD) and interstital lung disease (ILD) are incurable conditions characterised by airflow limitation, persisting respiratory symptoms, and progressive respiratory failure. People living with COPD or ILD often suffer from chronic and severe breathlessness, with limited treatment options and low engagement rates with current therapies. Group singing represents a potential community-based therapy to improve quality of life for patients with COPD or ILD and breathlessness. Methods This protocol papers describes SINFONIA, a parallel, double-arm, randomised, blinded-analysis, mixed-methods phase II/III trial of guided, online group singing that will be conducted over 24 months. Adults with confirmed COPD or ILD, on stable treatment for at least four weeks at time of recruitment, with a modified Medical Research Council (mMRC) dyspnoea score of two or greater, who are capable and willing to give consent, and not currently participating in pulmonary rehabilitation will be eligible to participate. Carers may optionally enrol in the trial. Data will be collected on quality of life, anxiety and depression, breathlessness, mastery of breathing, exercise tolerance, loneliness, healthcare utilisation, and carer quality of life (optional). Participants will be randomised 1:1 to intervention or control arms with intervention arm attending one 90 min, guided, online, group singing session per week for 12 weeks and control arm continuing routine care. Phase II of the trial aims to determine the feasibility and acceptability of guided, online group singing and will collect preliminary data on effectiveness. Phase III aims to determine whether guided, online group singing has an effect on quality of life with the primary outcome being a between arm difference in quality of life (36-item Short Form Survey) measured at 12 weeks. Discussion SINFONIA is the first study is the first of its kind in Australia and to our knowledge, the first to deliver the singing intervention program entirely online. Determining the feasibility, acceptability, and effectiveness of guided, online group singing is an important step towards improving low-cost, low-risk, community-based therapeutic options for patients living with COPD or ILD and breathlessness. Trial registration: Phase II—ACTRN12621001274864, registered 20th September 2021; Phase III—ACTRN12621001280897, registered 22nd September 2021.