Objective: Individuals with fragile X syndrome (FXS) present primarily with cognitive and social deficits in addition to symptoms of ADHD. The relationship between symptoms of ADHD, cognitive functioning, and social skills has never been explicitly studied. Method: Here, we analyzed both longitudinal (n = 70; Time 1: ages 6-18; Time 2: ages 15-26) and cross-sectional (n = 73; Time 2 only) data using hierarchical linear regression to assess how global intellectual functioning (IQ) and symptoms of ADHD influence social functioning in individuals with FXS. Results: We found that ADHD symptoms at Times 1 and 2 consistently predict social functioning in both males and females with FXS at Time 2. Conclusion: Our results suggest that addressing ADHD symptoms in childhood may have positive, long-term effects on the social functioning of adolescents and young adults with FXS.