The current study examined the effect of children's positive relationships and interactions with their teachers and the development of executive function (EF) skills in first grade. A primary objective was to examine externalizing behaviour problems (EBPs) as a potential moderator of the link between teacher-child relationships and interactions and EF skills. Participants for the study included 1,364 first-grade children (48.3% female, M age = 7.02 years, 80.4% White) drawn from the National Institute of Child Health and Human Development Study of Early Child Care and Youth Development. There was limited evidence suggesting the EBP moderates the relation between teacher--child relationship quality and EF skill development in first grade. Specifically, simple slope trends indicated that teacher-child closeness was positively related to gains in sustained attention for typically developing children but negatively related to sustained attention for children exhibiting borderline to clinical levels of EBPs. Implications of the findings are discussed.