OBJECTIVE: To determine the effectiveness of two online behavioural interventions, one for parents and carers and one for young people, to support eczema self-management. DESIGN: Two independent, pragmatic, parallel group, unmasked, randomised controlled trials. SETTING: 98 general practices in England. PARTICIPANTS: Parents and carers of children (0-12 years) with eczema (trial 1) and young people (13-25 years) with eczema (trial 2), excluding people with inactive or very mild eczema (≤5 on POEM, the Patient-Oriented Eczema Measure). INTERVENTIONS: Participants were randomised (1:1) using online software to receive usual eczema care or an online (www.EczemaCareOnline.org.uk) behavioural intervention for eczema plus usual care. MAIN OUTCOME MEASURES: Primary outcome was eczema symptoms rated using POEM (range 0-28, with 28 being very severe) every four weeks over 24 weeks. Outcomes were reported by parents or carers for children and by self-report for young people. Secondary outcomes included POEM score every four weeks over 52 weeks, quality of life, eczema control, itch intensity (young people only), patient enablement, treatment use, perceived barriers to treatment use, and intervention use. Analyses were carried out separately for the two trials and according to intention-to-treat principles. RESULTS: 340 parents or carers of children (169 usual care; 171 intervention) and 337 young people (169 usual care; 168 intervention) were randomised. The mean baseline POEM score was 12.8 (standard deviation 5.3) for parents and carers and 15.2 (5.4) for young people. Three young people withdrew from follow-up but did not withdraw their data. All randomised participants were included in the analyses. At 24 weeks, follow-up rates were 91.5% (311/340) for parents or carers and 90.2% (304/337) for young people. After controlling for baseline eczema severity and confounders, compared with usual care groups over 24 weeks, eczema severity improved in the intervention groups: mean difference in POEM score -1.5 (95% confidence interval -2.5 to -0.6; P=0.002) for parents or carers and -1.9 (-3.0 to -0.8; P<0.001) for young people. The number needed to treat to achieve a 2.5 difference in POEM score at 24 weeks was 6 in both trials. Improvements were sustained to 52 weeks in both trials. Enablement showed a statistically significant difference favouring the intervention group in both trials: adjusted mean difference at 24 weeks -0.7 (95% confidence interval -1.0 to -0.4) for parents or carers and -0.9 (-1.3 to -0.6) for young people. No harms were identified in either group. CONCLUSIONS: Two online interventions for self-management of eczema aimed at parents or carers of children with eczema and at young people with eczema provide a useful, sustained benefit in managing eczema severity in children and young people when offered in addition to usual eczema care. TRIAL REGISTRATION: ISRCTN registry ISRCTN79282252.