Background Tackling malnutrition is a global health priority, helping children both survive and thrive. Acute malnutrition (wasting) in infants aged under 6 months (u6m) is often neglected. Worldwide, some 8.5 million infants u6m are affected yet recent World Health Organization malnutrition guidelines highlight numerous evidence gaps on how to best manage them. To inform future research, policy and programming, we aimed to identify risk factors associated with infant u6m wasting. Methods We did secondary data analysis of nationally representative, cross sectional Demographic and Health Surveys conducted in the last 10 years. We compared wasted infants u6m (weight-for-length <−2 z-scores) vs. non-wasted (weight-for-length ≥−2 z-score). We used simple and adjusted (for infant age, sex, socio-economic status) logistic regression to calculate odds of wasting associated with risk factors spanning three broad categories: household-related; maternal-related; infant-related. Results We analysed 16,123 infants u6m from 20 countries. Multiple risk factors were statistically associated with wasting. These included: poverty (Odds ratio, OR 1.22 (95% CI [1.01–1.48], p = 0.04)); low maternal body mass index (adjusted OR 1.53(1.29–1.80, p < 0.001); small infant size at birth (aOR 1.32(1.10–1.58, p < 0.01)); delayed start of breastfeeding (aOR 1.31(1.13–1.51, p < 0.001)); prelacteal feed (aOR 1.34(1.18–1.53, p < 0.001)); recent history of diarrhoea (aOR 1.37(1.12–1.67, p < 0.01)); mother disempowered (experiences violence; does not make decisions about health issues; does not engage with health services such as antenatal care, does not give birth in a health facility). ‘Protective’ factors associated with significantly decreased odds of infant u6m wasting included: educated mother (OR 0.64(0.54–0.76, p < 0.001)); mother in work (OR 0.82(0.72–0.94, p < 0.01)); currently breastfed (aOR 0.62(0.42–0.91, p = 0.02)), exclusively breastfed (aOR 0.84(0.73–0.97, p = 0.02). Discussion Infant u6m wasting is a complex, multifactorial problem associated with many risk factors; knowing them will help shape international and national management strategies. Whilst our observational study cannot prove causation, many factors identified are biologically plausible and/or socially important. They should be considered when assessing and managing infants u6m. Although supporting breastfeeding is core to future interventions, this alone is unlikely to be sufficient; strategies should involve multiple sectors, beyond just health and nutrition. By noting our results, future intervention studies could focus resources and maximise chances of achieving impact.