The relationship between migration and child health in individual countries is well known, but the cross-national variation in this relationship is largely untested. Using Demographic and Health Survey (DHS) data from 52 medium and low income countries, this study examines the effect of rural–urban migration on infant mortality and whether its effect varies cross-nationally. A secondary objective is to determine whether there is a relationship between the time a child is born in the migration process and infant mortality. Hypotheses are developed on the basis of competing theories on the relationship between migration and health. There are modest, but significant cross-national effects of rural–urban migration on infant mortality, which were better revealed in the presence of family- and child-level variables. The results also show that the unadjusted effects of rural–urban migration are quite substantial, but were largely accounted for by family- and child-level factors including education, socioeconomic status (SES), marital status, birth order, maternal age at child’s birth, and inter-births intervals. The results largely point to a selection process, which is further confirmed by results showing that the hazards of infant death increase with length of urban residence. Programs that target increasing maternal education, improving household SES, and lengthening interbirth intervals would therefore greatly benefit child survival in less developed countries.