Fragile X syndrome (FXS), the leading heritable cause of intellectual disability, has a co-occurrence rate of autism spectrum disorder (ASD) estimated at ~60%. The onset and rates of motor development in FXS are slower relative to neurotypical development, and even more so in the context of co-occurring FXS + ASD. Extant evidence suggests these differences are likely to affect communication, yet this developmental process or how it varies in the context of co-occurring ASD remains unknown in FXS. We aimed to delineate developmental associations between early motor abilities and their rate of development from 9 to 60 months of age on communication outcomes in 51 children with FXS, 28 of whom had co-occurring ASD. We also aimed to identify variation in these developmental associations in the context of co-occurring ASD. Results captured within-syndrome variability in these developmental associations as a function of co-occurring ASD. Fine motor proved to be a robust predictor of receptive communication regardless of co-occurring ASD, but we identified differences between FXS with and without ASD in the association between aspects of motor development and expressive outcomes. Findings provide evidence for differential developmental processes in the context of co-occurring ASD with implications for timely developmental intervention.