ObjectiveWe aimed to understand the mortality risks of vulnerable newborns (defined as preterm and/or born weighing smaller or larger compared to a standard population), in low‐ and middle‐income countries (LMICs).DesignDescriptive multi‐country, secondary analysis of individual‐level study data of babies born since 2000.SettingSixteen subnational, population‐based studies from nine LMICs in sub‐Saharan Africa, Southern and Eastern Asia, and Latin America.PopulationLive birth neonates.MethodsWe categorically defined five vulnerable newborn types based on size (large‐ or appropriate‐ or small‐for‐gestational age [LGA, AGA, SGA]), and term (T) and preterm (PT): T + LGA, T + SGA, PT + LGA, PT + AGA, and PT + SGA, with T + AGA (reference). A 10‐type definition included low birthweight (LBW) and non‐LBW, and a four‐type definition collapsed AGA/LGA into one category. We performed imputation for missing birthweights in 13 of the studies.Main Outcome MeasuresMedian and interquartile ranges by study for the prevalence, mortality rates and relative mortality risks for the four, six and ten type classification.ResultsThere were 238 143 live births with known neonatal status. Four of the six types had higher mortality risk: T + SGA (median relative risk [RR] 2.8, interquartile range [IQR] 2.0–3.2), PT + LGA (median RR 7.3, IQR 2.3–10.4), PT + AGA (median RR 6.0, IQR 4.4–13.2) and PT + SGA (median RR 10.4, IQR 8.6–13.9). T + SGA, PT + LGA and PT + AGA babies who were LBW, had higher risk compared with non‐LBW babies.ConclusionsSmall and/or preterm babies in LIMCs have a considerably increased mortality risk compared with babies born at term and larger. This classification system may advance the understanding of the social determinants and biomedical risk factors along with improved treatment that is critical for newborn health.