Abstract Background Excess male morbidity and mortality is well recognised in neonatal medicine and infant health. In contrast, within global nutrition, it is commonly assumed that girls are more at-risk of experiencing undernutrition. We aimed to explore evidence for any male/female differences in child undernutrition using anthropometric case definitions and the reasons for differences observed. Methods We searched: Medline, Embase, Global health, Popline and Cochrane databases with no time limits applied. Eligible studies focused on children aged 0-59 months affected by undernutrition where sex was reported. In the meta-analysis, undernutrition-specific estimates were examined separately for wasting, stunting and underweight using a random effects model. Results 76 studies were identified: 46/76 studies were included in the meta-analysis. In 20 which examined wasting, boys had higher odds of being wasted than girls (pooled OR 1.26, 95% CI 1.13-1.40). 39 examined stunting: boys had higher odds of stunting than girls (pooled OR 1.31 95% CI 1.24-1.39). 25 explored underweight: boys had higher odds of being underweight than girls (pooled OR 1.19, 95% CI 1.07-1.32). There was some limited evidence that the female advantage indicating lower risk of stunting and underweight was weaker in South Asia than other parts of the world. 44/76 (58%) studies discussed possible reasons for boy/girl differences; 11/76 (14%) cited studies with similar findings with no further discussion; 21/76 (28%) had no sex difference discussion. 6/44 studies (14%) postulated biological causes, 21/44 (48%) social causes and 17/44 (38%) to a combination. Conclusion Our review indicates that undernutrition in children under 5 is more likely to affect boys than girls, though the magnitude of these differences varies and is more pronounced in some contexts than others. Future research should further explore reasons for these differences and implications for nutrition policy and practice. Key Questions What is already known? Undernutrition (wasting, stunting, and underweight) is a public health problem affecting millions of children aged under 5 years globally. Although higher neonatal and infant morbidity/mortality for boys is well described, little attention has been given to sex differences in the field of undernutrition due to an assumption that girls are very often disadvantaged over boys. What are the new findings? In most settings studied, undernutrition is more common among boys than girls, though the extent of these differences varies and is reversed in a few contexts. Both biological and social mechanisms have been proposed to be responsible for the observed differences as well as a combination of the two. What do the new findings imply? Greater awareness of actual sex differences is needed within the field of nutrition. While sex-specific data is routinely analysed and reported in nutrition surveys it should be used in nutrition programming to better identify and understand what differences exist. Analysis should assess if the sex balance in programme admissions is reflective of the population undernutrition burden. Further research is needed to understand the mechanisms that lead to sex and gender differences in undernutrition and their implications. Better epidemiological understanding is a priority, as is work to explore their consequent effects on morbidity and mortality.