--- - |2+ Background Children with autism spectrum disorder (ASD) often experience impairments in adaptive behavior. Methods Developmental trajectories of adaptive behavior in ASD were examined in children from high‐risk (siblings of children diagnosed with ASD, n = 403) and low‐risk (no family history of ASD, n = 163) families. Children were assessed prospectively at 12, 18, 24, and 36 months of age using the Vineland Adaptive Behavior Scales and underwent a blind independent diagnostic assessment for ASD at 36 months of age. Results The semi‐parametric group‐based modeling approach using standard scores on the Adaptive Behavior Composite revealed three distinct developmental trajectories: (a) Group 1 (21.2% of sample) showed average performance at 12 months and a declining trajectory; (b) Group 2 (52.8% of the sample) showed average performance at 12 months with a slightly declining trajectory; and (c) Group 3 (26.0% of the sample) showed a higher level of adaptive behavior at 12 months and a stable trajectory. The Mullen Scales of Early Learning Early Learning Composite and the Autism Observation Scale for Infants total score at 6 and 12 months predicted trajectory membership. Conclusions The results emphasize heterogeneous development associated with ASD and the need for interventions tailored to individual presentations. - Journal of Child Psychology and Psychiatry, EarlyView.