Background: Depression in adolescents is a common and impairing problem. Effective psychological therapies for depression are not accessed by most adolescents. Computerised therapy offers huge potential for improving access to treatment. Aims: To test the efficacy of Stressbusters, a Computerised-CBT (C-CBT) programme for depression in young people. Method: Multi-site, schools-based, RCT of C-CBT compared to Waiting List, for young people (N = 112; aged 12-16) with significant symptoms of depression, using multiple-informants (adolescents, parents, teachers), with follow-up at 3 and 6 months. Results: Relative to being on a Waiting List, C-CBT was associated with statistically significant and clinically meaningful improvements in symptoms of depression and anxiety according to adolescent self-report; and with a trend towards improvements in depression and anxiety according to parent-report. Improvements were maintained at follow-up. Treatment gains were similar for boys and girls across the participating age range. Treatment effect was partially mediated by changes in ruminative thinking. Teachers rated adolescents as having few emotional or behavioural problems, both before and after intervention. C-CBT had no detectable effect on academic attainment. In the month after intervention, young people who received C-CBT had significantly fewer absences from school than those on the Waiting List. Conclusions: C-CBT shows considerable promise for the treatment of mild-moderate depression in adolescents.