Purpose: In this study, we examine joint engagement (JE) in young children with fragile X syndrome (FXS) and its relationship to language abilities and autism spectrum disorder symptomatology at 24 to 36 months (toddler period) and 59 to 68 months (child period). Method: Participants were 28 children with FXS (24 boys, four girls) and their mothers. Videotaped home observations were conducted during the toddler period and coded for JE. Language abilities were measured at both ages from a developmental assessment, a functional measure, and from a language sample. The Childhood Autism Rating Scale (Schopler, Reichler, & Renner, 1988) was completed at both ages. Results: Children with FXS spent more time in supported JE than in coordinated JE. Using a weighted JE variable, we found that children with FXS who had higher weighted JE scores also had more advanced expressive language skills at both the toddler and child periods. Weighted JE was negatively related to autism symptomatology in the toddler period. Conclusion: This study provides evidence that children with FXS who use more JE also have more advanced expressive language skills in early development. Therefore, existing early interventions that target JE behaviors may be effective for promoting language, social communication, and social interaction in this population.