BACKGROUND: Injectable lenacapavir administered every 6 months is a promising product for HIV pre-exposure prophylaxis (PrEP). We aimed to estimate the health and budget impacts and threshold price at which lenacapavir could be cost-effective in eastern and southern Africa. METHODS: We adapted an agent-based network model, EMOD-HIV, to simulate lenacapavir scale-up in Zimbabwe, South Africa, and western Kenya from 2026 to 2035. Uptake assumptions were informed by a literature review of PrEP product preferences. In the main analysis, we varied lenacapavir coverage by subgroup: female sex workers (40% coverage); male clients of female sex workers (40%); adolescent girls and young women aged 15-24 years with more than one sexual partner (32%); women aged 25 years and older with more than one sexual partner (36%); and males with more than one sexual partner (32%). We also assessed a higher coverage scenario (64-76% across subgroups) and scenarios of expanding lenacapavir use, varying from concentrated among those at highest HIV risk to broader coverage including those at medium HIV risk. We estimated the maximum per-dose lenacapavir price that achieved cost-effectiveness (<US$500 per disability-adjusted life-year averted), infections averted, and 5-year budget impact, compared with daily oral PrEP only. FINDINGS: In the main analysis, lenacapavir was projected to achieve from 1·6% (95% uncertainty interval [UI] 1·5-1·8) to 4·0% (3·4-5·1) population coverage across settings and to avert from 12·3% (5·4-19·5) to 18·0% (11·0-22·9) of infections over 10 years. The maximum price per dose was highest in South Africa ($106·28 [95% UI 95·72-115·87]), followed by Zimbabwe ($21·15 [17·70-24·89]), and lowest in western Kenya ($16·58 [15·44-17·70]). The 5-year budget impact was US$507·25 million (95% UI 436·14-585·42) in South Africa, $16·80 million (13·95-22·64) in Zimbabwe, and $4·09 million (3·86-4·30) in western Kenya. In the higher coverage scenario, lenacapavir distribution was projected to reach from 3·2% (95% UI 2·9-3·6) to 8·1% (6·8-10·5) population coverage and to avert from 21·2% (95% UI 14·7-18·5) to 33·3% (28·5-36·9) of HIV infections across settings over 10 years. Price thresholds were lower than in the main analysis: $88·34 (95% UI 83·02-94·19) in South Africa, $17·71 (15·61-20·05) in Zimbabwe, and $14·78 (14·33-15·30) in western Kenya. The 5-year budget impact was higher than the main analysis: $835·29 million (95% UI 736·98-962·98) in South Africa, $29·50 million (24·62-39·52) in Zimbabwe, and $7·45 million (7·11-7·85) in western Kenya. Expanding lenacapavir coverage resulted in higher HIV infections averted but lower price thresholds than scenarios of concentrated use among those with highest HIV risk. INTERPRETATION: Our findings suggest that lenacapavir could avert substantial HIV incidence and that price thresholds and budget impacts vary by setting and coverage. These results could inform policy deliberations regarding lenacapavir pricing and resource planning. FUNDING: The Bill & Melinda Gates Foundation.