Effortful control, or the ability to suppress a dominant response to perform a subdominant response, is an early-emerging temperament trait that is linked with positive social-emotional development. Fragile X syndrome (FXS) is a single-gene disorder characterized by hallmark regulatory impairments, suggesting diminished effortful control. This study compared the development of effortful control in preschool boys with FXS (n = 97) and typical development (n = 32). Unlike their typical peers, the boys with FXS did not exhibit growth in effortful control over time, which could not be accounted for by adaptive impairments, FMR1 molecular measures, or autism symptoms. These results contribute to our understanding of the childhood phenotype of FXS that may be linked to the poor social-emotional outcomes seen in this group.