Fewer than 25% of patients with atherosclerotic cardiovascular disease in countries of low and middle income (LMICs) use guideline-directed drugs for secondary prevention. A fixed-dose combination polypill might improve cardiovascular outcomes by increasing prescription rates and adherence, but the cost-effectiveness of this approach is uncertain. We developed microsimulation models to assess the cost-effectiveness of a polypill containing aspirin, lisinopril, atenolol, and simvastatin for secondary prevention of atherosclerotic cardiovascular disease compared with current care in China, India, Mexico, Nigeria, and South Africa. We modelled baseline use of secondary prevention drugs on the Prospective Urban Rural Epidemiological study. In the intervention arm, we assumed that patients currently prescribed any prevention drug for atherosclerotic cardiovascular disease would receive the polypill instead, which would improve adherence by 32% (from a meta-analysis of two randomised trials in LMICs). We assessed the cost-effectiveness of the polypill at prices in the public sector and on the retail market. Key outcomes were major adverse cardiovascular events (cardiovascular death, non-fatal myocardial infarction, or non-fatal stroke) over a 5-year period and the incremental cost-effectiveness ratio (ICER) from the perspective of the health-care sector and a lifetime analytical horizon. We assumed a cost-effectiveness threshold equal to each country's per capita gross domestic product (GDP) per disability-adjusted life-year (DALY) averted. In sensitivity analyses, we examined the population health effect achievable by increasing the uptake of the polypill in the eligible population. Among adults aged 30-84 years with established atherosclerotic cardiovascular disease, adoption of the polypill for secondary prevention compared with current care was projected to avert 40-54 major adverse cardiovascular events for every 1000 patients treated for 5 years and produce between three and ten additional serious adverse events. Assuming public-sector pharmaceutical prices, the ICER of the polypill compared with current care over a lifetime analytical horizon was Int$168 (95% UI 55 to 337) per DALY averted in China, $154 (57 to 289) in India, $88 (15 to 193) in Mexico, $364 (147 to 692) in Nigeria, and $64 (cost-saving to 203) in South Africa, amounting to 0·4-6·2% of the per capita GDP in these countries. The ICER of the polypill compared with current care increased to 3·3-14·6% of the per capita GDP at retail market pharmaceutical prices. Use of the polypill at current rates of prescription of secondary prevention drugs would produce modest health benefits, reducing DALYs from atherosclerotic cardiovascular disease among patients with established disease by 3·1-10·1% over 10 years. Increasing use to 50% or 75% of the eligible population would produce substantially larger health gains (up to 24·3% atherosclerotic cardiovascular disease DALYs averted). The polypill is projected to be cost-effective compared with current care for secondary prevention of atherosclerotic cardiovascular disease in China, India, Mexico, Nigeria, and South Africa, particularly if it is made available at public-sector pricing. However, achieving meaningful improvements in cardiovascular health will require simultaneous investments in health infrastructure to increase the uptake of the polypill among patients with established atherosclerotic cardiovascular disease. Richard A and Susan F Smith Center for Outcomes Research in Cardiology, Hellman Family Foundation, Department of Veterans Affairs, and University of California at San Francisco.