BACKGROUND: Benefits to patients from reduced depression have been shown from monitoring progress with patient-reported outcome measures (PROMs) in psychological therapy and mental health settings. This approach has not yet been researched in the United Kingdom for primary care, which is where most people with depression are treated in the United Kingdom. METHODS: This is a parallel-group cluster randomised trial with 1:1 allocation to intervention and control. Patients who are age 18+ years, with a new episode of depressive disorder/symptoms, meet the inclusion criteria. Patients with current depression treatment, comorbid dementia/psychosis/substance misuse/suicidal ideas are excluded. The intervention includes the Administration of Patient Health Questionnaire (PHQ-9) as a PROM within 2 weeks of diagnosis and at follow-up 4 weeks later. General practitioners are trained in interpreting scores and asked to take them into account in their treatment decisions. Patients are given written feedback on scores and suggested treatments. The primary outcome measure is Depression on the Beck Depression Inventory BDI-II at 12 weeks. Secondary outcomes include BDI-II at 26 weeks, changes in drug treatments and referrals, social functioning (Work & Social Adjustment Scale) and quality of life (EQ-5D) at 12 and 26 weeks, service use over 26 weeks (modified Client Services Receipt Inventory) to calculate NHS costs, and patient satisfaction at 26 weeks (Medical Informant Satisfaction Scale). The sample includes 676 total participants from 113 practices across three centres. Randomisation is achieved by computerised sequence generation. Blinding is impossible given the nature of the intervention (self-report outcome measures prevent rating bias). Differences at 12 and 26 weeks between intervention and controls in depression, social functioning and quality of life are analysed using linear mixed models, adjusted for socio-demographics, baseline depression, anxiety, and clustering, while including practice as a random effect. Patient satisfaction, quality of life (QALYs) and costs over 26 weeks will be compared between arms. Qualitative process analysis includes interviews with 15-20 GP/NPs and 15-20 patients per arm to reflect trial results and implementation issues, using Normalization Process Theory as a theoretical framework. DISCUSSION: If PROMs are helpful in improving patient outcomes for depression even to a small extent, then they are likely to be good value for money, given their low cost. The benefits could be considerable, given that depression is common, disabling, and costly. TRIAL REGISTRATION: ISRCTN no: 17299295. Registered 1st October 2018.