This study assesses the interactions between sex, socioeconomic level, and children’s cognitive performance. Cognitive performance was measured for a sample of 453 Portuguese children, aged between 4 and 10 years, with 218 boys and 235 girls; verbal and nonverbal cognitive ability and intelligence quotient were measured by the Cognitive Skills Scale for Children. Multivariate analysis of variance assessed the effects of sex and family’s socioeconomic level on intelligence quotient. A statistically significant interaction between sex and socioeconomic level was observed for nonverbal intelligence quotient, total intelligence quotient, and two subtests. Socioeconomic level had more influence than sex on most of the cognitive tests.