Objectives To examine whether adverse childhood experiences (ACEs) and physical punishment (ie, spanking) are unique risk factors for behavior problems in early childhood, and whether ACEs moderate the associations of spanking with child behavior problems. Study design We conducted prospective, longitudinal analyses on 2380 families in the Fragile Families and Child Wellbeing Study. Mothers reported outcomes of externalizing and internalizing behavior problems at age 5 years; and the main predictors, ACEs and spanking, at age 3 years. ACEs included 9 items: physical abuse, emotional abuse, physical neglect, emotional neglect, mother's exposure to intimate partner violence, parental mental health problem, parental substance use, parental incarceration, and parental death. Multilevel models examined the associations between ACEs, spanking, and behavior problems, and the moderating effect of ACEs in the associations of spanking with behavior problems. Analyses were adjusted for preexisting behavior problems, demographics, and neighborhood conditions. Results ACEs (b = 0.028; P < .001) and spanking (b = 0.041; P < .001) at 3 years were unique risk factors for increased externalizing behavior problems at 5 years, after controlling for covariates. The magnitude of the associations of ACEs and spanking with externalizing behavior were statistically indistinguishable. ACEs did not moderate the association between spanking and externalizing behavior. Conclusions ACEs and spanking have similar associations in predicting child externalizing behavior. Results support calls to consider physical punishment as a form of ACE. Our findings also underscore the importance of assessing exposure to ACEs and physical punishment among young children and providing appropriate intervention to children at risk.