Background: Feedback-informed treatment (FIT) involves using computerized routine outcome monitoring technology to alert therapists to cases that are not responding well to psychotherapy, prompting them to identify and resolve obstacles to improvement. In this study, we present the first health economic evaluation of FIT, compared to usual care, to enable decision makers to judge whether this approach represents a good investment for health systems. Methods: This randomised controlled trial included 2233 patients clustered within 77 therapists who were randomly assigned to a FIT group (n = 1176) or a usual care control group (n = 1057). Treatment response was monitored using patient-reported depression (PHQ-9) and anxiety (GAD-7) measures. Therapists in the FIT group had access to a computerized algorithm that alerted them to cases that were “not on track”, compared to normative clinical data. Health service costs included the cost of training therapists to use FIT and the cost of therapy sessions in each arm. The incremental cost-effectiveness of FIT was assessed relative to usual care, using multilevel modelling. Results: FIT was associated with an increased probability of reliable symptomatic improvement by 8.09 percentage points (95% CI: 4.16%–12.03%) which was statistically significant. The incremental cost of FIT was £15.17 (95% CI: £6.95 to £37.29) per patient and was not statistically significant. The incremental cost-effectiveness ratio (ICER) per additional case of reliable improvement was £187.4 (95% CI: £126.7 to £501.5); this confidence interval shows that the relative cost-effectiveness is between FIT being a dominant strategy (i.e. more effective and also cost-saving) to FIT being more effective at a modest incremental cost to the health system. Conclusions: The FIT strategy increases the probability of reliable improvement in routine clinical practice and may be associated with a small (but uncertain) incremental cost. FIT is likely to be a cost-effective strategy for mental health services.