Key PointsQuestionAre preterm birth, socioeconomic status (SES), and neonatal brain structure associated?FindingsIn this cohort study of 261 infants, after mutual adjustment, both low birth gestational age (GA) and SES were associated with brain structure. The nature of SES–brain structure associations varied depending how SES was operationalized; there were interactions between GA and measures of family-level SES on brain structure.MeaningIn this study, low birth GA, and to a lesser extent SES, were associated with neonatal brain structure; further work is required to elucidate potential mechanisms underlying this association.