Assisted partner services (APS) is a recommended public health approach to promote HIV testing for sexual partners of individuals diagnosed with HIV. We evaluated the cost and cost-effectiveness of integrating oral HIV self-testing (HIVST) into existing APS programs. Within the APS-HIVST study conducted in western Kenya (2021-2022), we conducted micro-costing, time-and-motion, and provider surveys to determine incremental HIVST distribution cost (2022 USD). Using a decision tree model, we estimated the incremental cost per new diagnosis (ICND) for HIVST incorporated into APS, compared to APS with provider-delivered testing only. Scenario, parameter and probabilistic sensitivity analyses were conducted to explore influential assumptions. The cost per HIVST distributed within APS was USD 8.97, largest component costs were testing supplies (38%) and personnel (30%). Under conditions of a facility-based testing uptake of <91%, or HIVST utilization rates of <27%, HIVST integration into APS is potentially cost-effective. At a willing-to-pay threshold of USD 1000, the net monetary benefit was sensitive to the effectiveness of HIVST in increasing testing rates, phone call rates, HIVST sensitivity, HIV prevalence, cost of HIVST, space allocation at facilities, and personnel time during facility-based testing. In a best-case scenario, the HIVST option was cheaper by USD 3037 and diagnosed 11 more cases (ICND = 265.82). Implementers and policy makers should ensure that HIVST programs are implemented under conditions that guarantee efficiency by focusing on facilities with low uptake for provider-delivered facility-based testing, while deliberately targeting HIVST utilization among the few likely to benefit from remote testing. Additional measures should focus on minimizing costs relating to personnel and testing supplies.