INTRODUCTION: Poor retention (missed appointments or treatment discontinuation) on antiretroviral therapy (ART) may lead to disease progression, increased mortality, and HIV transmission. This study quantified the financial costs of implementing 12 months of routine services to improve ART retention at a large public clinic in Lilongwe, Malawi. METHODS: We performed activity-based micro-costing from the payer perspective using routine program data to assess the costs associated with two retention interventions: ART treatment buddies (expert clients) and Back-to-Care (B2C) tracing services. New ART initiates receive a Buddy who provides proactive retention support for 12 months. B2C traces all clients who miss ART visits by ≥ 14 days. Costs were calculated as total and per-client expenditures, reported in 2021 USD. RESULTS: The total cost for 12 months of ART retention activities was 37,564. Buddies accounted for 08,504, with personnel costs contributing 7,764. B2C totaled 29,060, with personnel expenses remaining substantial at 3,778. The unit costs were 4 per client Buddy and 7 per B2C tracing event. CONCLUSION: This study highlights the financial costs of delivering 12 months of ART retention services at a public, low-resource clinic. While proactive and reactive retention strategies are costly, both are essential for engaging ART clients from initiation onward.