["Autism Research, EarlyView. ", "\nABSTRACT\nHigh levels of parental stress and psychological inflexibility are common among caregivers raising children with neurodevelopmental disabilities. Navigator ACT is a group‐based treatment developed to increase psychological flexibility and reduce the impact of stress among parents of children with various disabilities (e.g., autism spectrum disorder, ADHD, intellectual disability, acquired brain injury). This two‐arm randomized controlled trial (n = 137) was conducted in Sweden within outpatient disability services setting, and pre‐registered in the clinical trials register. To compare conditions, stressed and distressed parents were randomly assigned to either the ACT group (n = 70) or treatment‐as‐usual (TAU, n = 67). In the ACT group, 83.3% completed the full course of treatment. Mixed‐model linear regression analyses indicated that ACT was significantly more effective than TAU in reducing self‐reported psychological inflexibility (p < 0.002, d = 0.84) and parenting stress (p < 0.001, d = 0.38). These improvements were maintained at four‐month follow‐up. Parents in the ACT group also reported significantly greater improvements in their children's prosocial behaviors (p < 0.05, d = 0.46). However, no significant group differences were observed in levels of parental depression, anxiety, mindfulness, or child's difficulties. ACT group treatment demonstrated promising outcomes in reducing psychological inflexibility and parenting stress in a mixed group of parents of children with different neurodevelopmental disabilities, suggesting that parent support interventions do not need to be specific to the child's diagnosis.\n"]