Background: Previous research suggests that the use of outcome feedback technology can enable psychological therapists to identify and resolve obstacles to clinical improvement. We aimed to assess the effectiveness of an outcome feedback quality assurance system applied in stepped care psychological services. Methods: This multisite, open-label, cluster randomised controlled trial was done at eight National Health Service (NHS) Trusts in England, involving therapists who were qualified to deliver evidence-based low-intensity or high-intensity psychological interventions. Adult patients (18 years or older) who accessed individual therapy with participating therapists were eligible for inclusion, except patients who accessed group therapies and those who attended less than two individual therapy sessions. Therapists were randomly assigned (1:1) to an outcome feedback intervention group or a treatment-as-usual control group by use of a computer-generated randomisation algorithm. The allocation of patients to therapists was quasi-random, whereby patients on waiting lists were allocated sequentially on the basis of therapist availability. All patients received low-intensity (less than eight sessions) or high-intensity (up to 20 sessions) psychological therapies for the duration of the 1-year study period. An automated computer algorithm alerted therapists in the outcome feedback group to patients who were not on track, and primed them to review these patients in clinical supervision. The primary outcome was symptom severity on validated depression (Patient Health Questionnaire-9 [PHQ-9]) and anxiety (Generalised Anxiety Disorder-7 [GAD-7]) measures after treatment of varying durations, which were compared between groups with multilevel modelling, controlling for cluster (therapist) effects. We used an intention-to-treat approach. This trial was prospectively registered with ISRCTN, number ISRCTN12459454. Findings: In total, 79 therapists were recruited to the study between Jan 8, 2016, and July 15, 2016, but two did not participate. Of these participants, 39 (51%) were randomly assigned to the outcome feedback group and 38 (49%) to the control group. Overall, 2233 patients were included in the trial (1176 [53%] were treated by therapists in the outcome feedback group, and 1057 [47%] by therapists in the control group). Patients classified as not on track had less severe symptoms after treatment if they were allocated to the outcome feedback group than those in the control group (PHQ-9 d=0·23, B=–1·03 [95% CI −1·84 to −0·23], p=0·012; GAD-7 d=0·19, B=–0·85 [–1·56 to −0·14], p=0·019). Interpretation: Supplementing psychological therapy with low-cost feedback technology can reduce symptom severity in patients at risk of poor response to treatment. This evidence supports the implementation of outcome feedback in stepped care psychological services. Funding: English NHS and Department of Health Sciences, University of York, York, UK.