Background Suicide-related behaviours are common in young people and associated with a range of negative outcomes. There are few evidencebased interventions; however, cognitive behavioural therapy (CBT) shows promise. Internet delivery of CBT is popular, with potential to increase reach and accessibility. Objective To test the effectiveness of an internet-based CBT program (Reframe-IT) in reducing suicide-related behaviours, depression, anxiety, hopelessness and improving problem solving and cognitive and behavioural skills in school students with suicide-related behaviours. Methods A parallel randomised controlled trial testing the effectiveness of Reframe-IT plus treatment as usual (TAU) compared with TAU alone in reducing suicidal ideation, suicide attempts, depression, hopelessness, symptoms of anxiety, negative problem orientation and cognitive and behavioural skill acquisition was undertaken. We recruited students experiencing suicidal ideation from 18 schools in Melbourne, Australia, between August 2013 and December 2016. The intervention comprised eight modules of CBT delivered online over 10 weeks with assessments conducted at baseline, 10 weeks and 22 weeks. Findings Only 50 of the planned 169 participants were recruited. There were larger improvements in the Reframe-IT group compared with the TAU group for the primary outcome of suicidal ideation (intervention −61.6, SD 41.6; control −47.1, SD 42.3, from baseline to 22-week follow-up intervention); however, differences were non-significant (p=0.593). There were no increases in distress in the majority of participants (91.1%) after completion of each module. Changes in depression and hopelessness partly mediated the effect of acquisition of CBT skills on suicidal ideation. Conclusions The trial was underpowered due to difficulties recruiting participants as a result of the complex recruitment procedures that were used to ensure safety of participants. Although there were no significant differences between groups, young people were safely and generally well engaged in Reframe-IT and experienced decreases in suicidal ideation and other symptoms as well as improvements in CBT skills. The study is the first online intervention trial internationally to include young people demonstrating all levels of suicide risk. Clinical implications Integration of internet-delivered interventions for young people with suicide-related behaviour may result in reductions in these behaviours. Further research is needed, but researchers should feel more confident about being able to safely undertake research with young people who experience these behaviours.