Disordered sleep is common in autistic children. This study aimed to evaluate the cost-effectiveness of a brief behavioural sleep intervention, the 'Sleeping Sound intervention', in primary school-aged autistic children in Australia. A cost-effectiveness analysis was undertaken alongside a randomised controlled trial over a 6-month follow-up period from both a societal and healthcare sector perspective. Resources used by participants were collected from a resource-use questionnaire and administrative data; intervention costs were determined from study records. Mean costs and quality-adjusted life-years (QALYs) were compared between the intervention and treatment as usual (TAU) groups. Uncertainty analysis using bootstrapping and sensitivity analyses were conducted. The sample included 245 children, with 123 participants randomised to the intervention group and 122 to TAU. The mean total costs were higher for the "Sleeping Sound" intervention with a mean difference of A$745 (95% CI 248; 1242; p = 0.003) from a healthcare sector perspective and A$1310 (95% CI 584; 2035, p < 0.001) from a societal perspective. However, the intervention also resulted in greater QALYs compared with TAU, with a mean difference of 0.038 (95% CI 0.004; 0.072; p = 0.028). The incremental cost-effectiveness ratio was A$24,419/QALY (95% CI 23,135; 25,703) from a healthcare sector perspective and A$41,922/QALY (95% CI 39,915; 43,928) from a societal perspective; with a probability of being cost-effective of 93.8% and 74.7%, respectively. Findings remained robust in the sensitivity analyses. The "Sleeping Sound" intervention offers a cost-effective approach in improving sleep in primary school-aged autistic children.