Emerging evidence suggests that children with fragile X syndrome (FXS) exhibit abnormal gesture use early in development, although few studies have investigated the emergence of gesture use in this population or the impact of autism spectrum disorder (ASD) features on these behaviours. The present study examined the longitudinal development of gesture use in infants with FXS relative to low-risk controls and infant siblings of children with ASD (high-risk siblings), with the goal of establishing potentially unique patterns of gesture development in infants with FXS and understanding the relative impact of ASD symptom severity on these patterns. Participants included 86 male infants (39 FXS, 27 high-risk siblings and 20 low-risk infants) assessed at 9, 12 and 24 months of age. Multilevel modelling was used to assess differences in number of gestures used and rates of gesture use across groups, as well as the relative impact of ASD symptom severity and nonverbal skills on these patterns. Infants with FXS used fewer gestures than high-risk siblings and low-risk infants, with this difference being primarily accounted for by the effect of low nonverbal abilities in the FXS group. Furthermore, although higher ASD symptom severity was associated with the use of fewer gestures in both the FXS and high-risk sibling groups, a significant amount of variance was shared between ASD symptom severity and nonverbal skills in FXS, but not in high-risk siblings. This study presents the first longitudinal analysis of early gesture development in FXS by using a multigroup design, clarifying the relative roles of cognitive deficits and ASD symptom severity in the development of gesture use in FXS. These findings offer novel evidence that early gesture use in FXS may reflect broader features of the FXS phenotype rather than predicting later social-communicative deficits characteristic of comorbid ASD. © 2018 MENCAP and International Association of the Scientific Study of Intellectual and Developmental Disabilities and John Wiley & Sons Ltd.