Behavioural interventions for paediatric obesity are promising, but detailed information on treatment fidelity (i.e. design, training, delivery, receipt and enactment) is needed to optimize the implementation of more effective interventions. Little is known about current practices for reporting treatment fidelity in paediatric obesity studies. This systematic review, in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, describes the methods used to report treatment fidelity in randomized controlled trials. Treatment fidelity was double-coded using the National Institutes of Health Fidelity Framework checklist. Three hundred articles (N = 193 studies) were included. Mean inter-coder reliability across items was 0.83 (SD = 0.09). Reporting of treatment design elements within the field was high (e.g. 77% of studies reported designed length of treatment session), but reporting of other domains was low (e.g. only 7% of studies reported length of treatment sessions delivered). Few reported gold standard methods to evaluate treatment fidelity (e.g. coding treatment content delivered). General study quality was associated with reporting of treatment fidelity (p < 0.01) as was the number of articles published for a given study (p < 0.01). The frequency of reporting treatment fidelity components has not improved over time (p = 0.26). Specific recommendations are made to support paediatric obesity researchers in leading health behaviour disciplines towards more rigorous measurement and reporting of treatment fidelity.