BACKGROUND: Malaria remains a major health burden in sub-Saharan Africa, where traditional vector control methods are hindered by insecticide resistance and evolving mosquito behaviour causing residual transmission. In the BOHEMIA cluster-randomised trial in Kenya, ivermectin mass drug administration (iMDA), delivered once a month for 3 months with approximately 64% population coverage, was shown to reduce malaria incidence by 26%. We aimed to assess the cost-effectiveness of iMDA as a supplementary vector control tool using data from the BOHEMIA trial in Kenya. METHODS: We did a cost-effectiveness analysis of the BOHEMIA cluster-randomised trial done in Kwale county, Kenya, using a societal perspective to estimate the intervention costs, health system costs, direct household out-of-pocket expenses, and indirect costs from lost wages of iMDA versus a no-intervention scenario. Intervention effectiveness was measured as the number of malaria cases averted and disability-adjusted life-years (DALYs) averted. A decision tree model was developed to simulate the intervention's impact on a broader population. Deterministic and probabilistic sensitivity analyses were performed to assess the robustness of the results, and incremental cost-effectiveness ratios (ICERs) were compared with Kenya's gross domestic product (GDP)-based thresholds. FINDINGS: The intervention cost of iMDA was US$11·83 per person. Household out-of-pocket costs averaged $5·85 for uncomplicated malaria cases and $52·23 for severe cases. Productivity loss amounted to $2·18 for uncomplicated and $8·83 for severe cases. The base-case ICER was $905·23 per DALY averted, which was below the threshold of 0·5 × Kenya's GDP per capita ($974·65). In probabilistic analysis (10 000 iterations), the median ICER was $1107·51 per DALY averted (50% credible interval 770·05-1606·77). INTERPRETATION: This study demonstrates that iMDA can be a cost-effective supplementary intervention for malaria control in settings with moderate malaria transmission and good insecticide-treated net coverage, particularly when malaria reduction is greater than 23·62% for children younger than 5 years and opportunities for reducing intervention costs can be identified. FUNDING: This work was funded and supported by Unitaid through the BOHEMIA project.