Sex differences in early child development (ECD) are well documented, as is the socioeconomic status (SES) gradient in early development outcomes. However, relatively little is known about whether the SES gradient in ECD outcomes varies by sex. This study examines whether the association between neighbourhood SES and developmental health outcomes of Canadian kindergarten children is different for girls than for boys. Individual-level child development data, collected using the Early Development Instrument (EDI), were combined with neighbourhood-level socioeconomic data from Statistics Canada's Census and Tax Filer databases. Using an SES index comprising 10 socioeconomic variables, we show a significant cross-level interaction between neighbourhood SES and sex in relation to children's developmental outcomes: the neighbourhood SES gradient in child outcomes is steeper for males than for females. This finding was consistent across all five developmental domains measured by the EDI, for overall developmental health, and across geographical regions in Canada. Further research using family-level SES data, data from multiple time points and countries, and qualitative studies would help to further contextualize the observed interactions.