BackgroundContextual factors, especially where people live, has been linked to various health outcomes, therefore, there is an increasing focus on its implication for policies and implementation of health interventions. Polygyny is a widespread practice in sub-Saharan Africa that also reflects socioeconomic and sociocultural features. This study investigated the association between polygynous context and risk of undernutrition.MethodsRecent Demographic and Health Surveys involving 350 000 mother–child pairs from 32 sub-Saharan African countries conducted between 2010 and 2018 as of March 2020, were analysed using relevant descriptive and 3-level multilevel logistic regression modelling. Undernutrition among under-5 was defined as underweight, stunting and wasting using the WHO Multicentre Growth Reference Study. Odd Ratio (OR) at 95% credible interval was used to report the associations.ResultsThe prevalence of contextual polygyny varied widely across the 32 sub-Saharan African countries, the lowest (0%) found in one of the regions in South Africa and the highest (52%) in one of the regions in Uganda. Underweight, stunting and wasting were lowest in Uganda (3.5%, 9.3%–1.27%, respectively), stunting was highest in Mozambique (37.1%) while wasting was highest in Niger (7.7%). Furthermore, the results showed that the contextual prevalence of polygynous practice exacerbates the risk of underweight (1.003 (0.997–1.008)) and wasting (1.014 (1.007–1.021)) among under-5 children, even when gender inequality and sociodemographic indicators were adjusted for. Polygyny was negatively associated with stunting though not significant; multiple births had the strongest and positive association with the risk of undernutrition among under-5 children in sub-Saharan Africa.ConclusionsThis study further corroborates the strong influence of contextual factors on health outcomes—which is undernutrition in this study. In addition to specific interventions aimed at reducing the prevalence of undernutrition, broader strategies that will address contextual issues are required.