Background: Prior self-harm represents the most significant risk factor for future self-harm or suicide. Aim: To evaluate the cost-effectiveness of a theoretical brief aftercare intervention (involving brief follow-up contact, care coordination and safety planning), following a hospital-treated self-harm episode, for reducing repeated self-harm within the Australian context. Method: We employed economic modelling techniques to undertake: (a) a return-on-investment analysis, which compared the cost-savings generated by the intervention with the overall cost of implementing the intervention; and (b) a cost–utility analysis, which compared the net costs of the intervention with health outcomes measured in quality-adjusted life years (QALYs). We considered cost offsets associated with hospital admission for self-harm and the cost of suicide over a period of 10 years in the base case analysis. Uncertainty and one-way sensitivity analyses were also conducted. Results: The brief aftercare intervention resulted in net cost-savings of AUD$7.5 M (95% uncertainty interval: −56.2 M to 15.1 M) and was associated with a gain of 222 (95% uncertainty interval: 45 to 563) QALYs over a 10-year period. The estimated return-on-investment ratio for the intervention's modelled cost in relation to cost-savings was 1.58 (95% uncertainty interval: −0.17 to 5.33). Eighty-seven per cent of uncertainty iterations showed that the intervention could be considered cost-effective, either through cost-savings or with an acceptable cost-effectiveness ratio of 50 000 per QALY gained. The results remained robust across sensitivity analyses. Conclusions: A theoretical brief aftercare intervention is highly likely to be cost-effective for preventing suicide and self-harm among individuals with a history of self-harm.