Purpose: Autistic children are reported to display higher levels of externalizing and internalizing behaviors than neurotypical children, and their parents report more stress than parents of neurotypical children. It is unclear whether child behavior difficulties contribute to increased parenting stress, whether parenting stress contributes to child behavior difficulties, or whether the relationship may be bidirectional. Methods: We investigated prospective bidirectional associations between parenting stress and child externalizing and internalizing behaviors when autistic children were aged on average 3.5, 4.5, and 5.5 years. Data collected at these three timepoints were examined across two panels: Time 1 to Time 2 (n = 38 parent-child dyads) and Time 2 to Time 3 (n = 27 dyads). Results: Across Time 1 to Time 2, early parenting stress was significantly associated with later child externalizing behavior, and cross-lagged panel analysis supported a uni- rather than a bidirectional association between these factors. There was some evidence of a bidirectional association between parenting stress and child internalizing behavior, though this was non-significant when the strong stability of child internalizing behavior was statistically controlled. In contrast, across Time 2 to Time 3, there were no significant prospective associations found between variables, highlighting the importance of considering the impact of parenting stress early in the course of childhood autism. Conclusion: Our results add to research indicating that support targeting parent characteristics, especially parenting stress, could ameliorate subsequent outcomes for both parents and children.