Being born preterm and being raised in poverty are each linked with adverse cognitive outcomes. Using data from 5,250 singletons born in the United States in 2001 and enrolled in the Early Childhood Longitudinal Study, Birth Cohort, we examined whether household socioeconomic status (SES) modified the association between preterm birth (PTB) and children's scores on cognitive assessments at age 2 years and reading and mathematics assessments at kindergarten age. Gestational age was measured from birth certificates and categorized as early preterm, moderate preterm, late preterm, early term, and term. SES was measured at age 9 months using a composite of parental education, occupation, and income. PTB was associated with 0.1- to 0.6-standard deviation-deficits in 2-year cognitive ability and kindergarten mathematics scores and with 0.1- to 0.4-standard-deviation deficits in kindergarten reading scores. Children living in the lowest (versus highest) SES quintile scored 0.6 standard deviations lower on 2-year cognitive ability, 1.1 standard deviations lower on kindergarten reading, and 0.9 standard deviations lower on kindergarten mathematics. The association between PTB and cognitive outcomes did not differ by postnatal SES. However, children who were both born preterm and lived in lower-SES households had the poorest performance on all 3 outcomes and therefore may represent a uniquely high-risk group.