OBJECTIVES: Much of our understanding about the effect of concurrent sexual partnerships on the spread of HIV derives from mathematical models, but the empirical evidence is limited. In this contribution, we focus on polygyny, a common and institutionalized form of concurrency for which data are available, and study its relationship with HIV prevalence at the ecological level. METHODS: First, we describe country-level variation in the prevalence of polygyny and HIV. Second, we test the relationship between HIV and polygyny at the subnational level using country fixed-effects regression models with data from 19 Demographic and Health Surveys. RESULTS: The ecological association between polygyny and HIV prevalence is negative at the country as well as subnational level, HIV prevalence is lower in countries where the practice of polygyny is common, and within countries, it is lower in areas with higher levels of polygyny. Proposed explanations for the protective effect of polygyny include the distinctive structure of sexual networks produced by polygyny, the disproportionate recruitment of HIV-positive women into marriages with a polygynous husband, and the lower coital frequency in conjugal dyads of polygynous marriages. CONCLUSION: Existing mathematical models of concurrency are not sufficiently specific to account for the relatively benign effect of polygyny on the spread of HIV and require refinements before they are used to inform HIV prevention policies.