Moderately preterm (MP) birth is associated with higher rates of behavioural and emotional problems. To determine the extent to which low socioeconomic status (SES) contributes to these higher rates, we assessed independent and joint effects of MP birth and low SES, overall and by gender. Dutch preventive child health care centres provided a population-based sample of 915 MP children (32-36 weeks gestation) and 543 term-born children, born in 2002/2003. In multivariable logistic regression analyses, we determined the risk of behavioural and emotional problems per standard deviation (SD) decrease in gestational age and SES, using standardized measures for both. We also assessed three SES categories, being low (1SD or more below mean of standardized SES), intermediate (mean ± 1SD), and high (greater than mean + 1SD). The Child Behavior Checklist for 1.5-5 years was used to assess behavioural (externalizing), emotional (internalizing), and total problems at age 4 years. MP children with low SES had significantly higher total problem scores than those with high SES (11.3 vs. 5.1%, respectively). Each SD decrease in SES was associated with a 42% higher odds of elevated total problem scores (OR 1.42, 95% CI 1.14-1.77). No joint effects were found, meaning that lower gestational age independently added to the risk of behaviour problems (OR 1.24, 95% CI 1.00-1.56). Effects of MP birth and low SES were more pronounced in girls. In conclusion, MP birth and low SES multiply the risk of behavioural and emotional problems. The combination of risk factors identifies children who could benefit greatly from early intervention.