Abstract Background The Relatives Education And Coping Toolkit (REACT) is an online supported self-management toolkit for relatives of people with psychosis or bipolar designed to improve access to NICE recommended information and emotional support. Aims Our aim was to determine clinical and cost-effectiveness of REACT including a Resource Directory (RD), versus RD-only. Methods A primarily online, observer-blind randomised controlled trial comparing REACT (including RD) with RD only (registration ISRCTN72019945). Participants were UK relatives aged > = 16, with high distress (assessed using the GHQ-28), and actively help-seeking, individually randomised, and assessed online. Primary outcome was relatives’ distress (GHQ-28) at 24 weeks. Secondary outcomes were wellbeing, support, costs and user feedback. Results We recruited 800 relatives (REACT = 399; RD only = 401) with high distress at baseline (GHQ-28 REACT mean 40.3, SD 14.6; RD only mean 40.0, SD 14.0). Median time spent online on REACT was 50.8 min (IQR 12.4–172.1) versus 0.5 min (IQR 0–1.6) on RD only. Retention to primary follow-up (24 weeks) was 75% (REACT n = 292 (73.2%); RD-only n = 307 (76.6%)). Distress decreased in both groups by 24 weeks, with no significant difference between the two groups (− 1.39, 95% CI -3.60, 0.83, p = 0.22). Estimated cost of delivering REACT was £62.27 per person and users reported finding it safe, acceptable and convenient. There were no adverse events or reported side effects. Conclusions REACT is an inexpensive, acceptable, and safe way to deliver NICE-recommended support for relatives. However, for highly distressed relatives it is no more effective in reducing distress (GHQ-28) than a comprehensive online resource directory. Trial registration ISRCTN72019945 prospectively registered 19/11/2015.