Background: The relationships between adaptive behavior (such as practical skills in daily life), cognitive functioning (such as intelligence quotient [IQ]), and autistic traits in the general population are not well researched. Methods: This study included 835 nine-year-old children from the Hamamatsu Birth Cohort Study for Mothers and Children (HBC Study). The associations between adaptive behavior and cognitive functioning were examined by IQ categories, levels of autistic traits, and their combinations using multiple regression analysis. Results: The association was positive for the total sample. The magnitude of the association weakened as IQ increased, but it was similar between children with lower and higher levels of autistic traits. When IQ and autistic traits were combined, the association was positive only in children with lower levels of autistic traits and within lower-than-average and average IQs. The predicted adaptive behavior score for cognitive functioning was approximately 10 points lower for those with higher levels of autistic traits than lower levels of autistic traits. Conclusion: Although the magnitude of the association was similar between children with lower and higher levels of autistic traits, those with higher levels of autistic traits exhibited lower adaptive behavior, highlighting the need for tailored support in this group.