<p>Many autistic children have spoken vocabulary delays that impact communication abilities during critical developmental periods. Behavioral and brain predictors of spoken language development in autism have been independently identified but rarely evaluated simultaneously. Here, we examined how fine motor skills, joint attention, and white matter microstructure of speech-related tracts are associated with spoken vocabulary development in autistic children with a broad range of speech abilities. Spoken vocabulary was assessed longitudinally in 122 autistic children (38% female) first between 2–4 years (Time 1) and again approximately two years later. Children were divided into groups based on their Time 1 spoken vocabularies: larger, average, or smaller. Fifty-eight autistic children (47% of the sample) were in the smaller vocabulary group, which represented both emergent speakers and non-speaking individuals. Within this smaller vocabulary group, faster rates of spoken vocabulary development were associated with better Time 1 fine motor and joint attention skills, and lower Time 1 fractional anisotropy (FA) in the occipital inferior longitudinal fasciculus, central arcuate fasciculus, and inferior corticospinal tract. Rates of spoken vocabulary development were best predicted by models that included both brain and behavior metrics, which each individually explained significant variance and together accounted for roughly half of the model variability. Results suggest that together early childhood fine motor skills, joint attention skills, and white matter microstructure contribute to a developmental cascade supporting spoken language. Findings demonstrate the potential clinical utility of early brain scanning, indicating that microstructure is predictive of language development above and beyond salient behavioral predictors.</p>