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Introduction Increasing handwashing with soap (HWWS) among older children in emergency settings can have a large public health impact, however, evidence on what works is limited. One promising approach is the ‘Surprise Soap’ intervention in which a novel soap with an embedded toy is delivered to children in a short, participatory household session that includes a glitter game and HWWS practice. Here, we evaluate this intervention against a standard intervention in a complex emergency setting.Methods A cluster-randomised controlled equivalence trial was conducted in Naivasha refugee settlement, Sudan. Blinding was not possible. 203 randomly selected households, with at least one child aged 5–12, were randomised to receive the Surprise Soap intervention (n=101) or a standard intervention comprising a short household session with health messaging and plain soap distribution (n=102). The primary outcome was the proportion of prespecified potential HWWS events observed for children aged 5–12, accompanied by HWWS, at baseline, 4, 12 and 16 weeks post intervention delivery.Results 200 households were included in the analyses: 101 intervention and 99 control. No difference in intervention effectiveness was observed at any follow-up (4 weeks: adjusted rate ratio (RR) 1.2, 95% CI 0.8 to 1.7; 12 weeks: RR 0.8, 95% CI 0.5 to 1.1; 16 weeks: RR 1.1, 95% CI 0.8 to 1.5). However, we observed increased HWWS in both arms at 4 weeks (27 and 23 percentage point increase in the intervention and control arm, respectively) that was sustained at 16 weeks.Conclusions We find that the Surprise Soap intervention is no more effective at increasing older children’s HWWS than a standard, household-level, health-based intervention in this complex humanitarian emergency. There appears to be no marginal benefit in terms of HWWS that would justify the additional cost of implementing the Surprise Soap intervention. Further trials that include a passive control arm are needed to determine the independent effects of each intervention and guide future intervention design.