This study assesses the causal effect of child marriage on infant mortality. Using age discontinuities in exposure to a law that raised the legal age of marriage for women in Ethiopia, the study estimates that a 1-year delay in a woman's age at cohabitation during her teenage years reduces the probability of her first-born child dying during infancy by 3.8 percentage points. This impact is closely linked to the effect of delaying cohabitation on women's age at first birth.