Purpose <p>Young autistic children (i.e., under 6 years old) are more likely to engage in frequent and severe aggression compared to non-autistic peers. Yet limited studies have examined aggression or its predictors in this age range. This cross-sectional study examined if intolerance of uncertainty (IU) and sensory over-reactivity (SOR), two mechanisms underlying internalizing conditions in autistic youth, are also associated with aggression in young, autistic children.</p> Methods <p>Data were analyzed from 59 autistic children aged 3 to 5 years-old from a larger study of anxiety phenomenology and correlates. Parents completed the Response to Uncertainty and Low Environmental Stability Scale (RULES), Short Sensory Profile-2 (SSP-2), and the Child Behavior Checklist (CBCL)—Aggressive Behavior Subscale to assess IU, SOR, and aggression, respectively. Child anxiety, which was examined as a potential confounding variable, was assessed using the Anxiety Disorders Interview Schedule-Parent Version with Autism Spectrum Addendum (ADIS/ASA).</p> Results <p>Multiple regression analysis revealed that IU and SOR were each positively associated with aggression while controlling for child anxiety and age. Although IU and SOR were moderately correlated, dominance analysis indicated they had moderate and relatively equivalent independent associations with aggression.</p> Conclusions <p>Findings emphasize the need for further research investigating IU and SOR as predictors of aggression in young autistic children and as promising targets for aggression intervention and prevention.</p>