Aim: To investigate the efficacy of psychological interventions delivered to mothers of young children with autism spectrum disorder (ASD) in improving maternal mental health and the parent–child relationship. Method: Electronic databases were searched from inception to December 2021. Studies were included if they were randomized controlled trials (RCTs) of psychological interventions that targeted maternal mental health (primary outcome) and/or the parent–child relationship (secondary outcome), delivered to mothers of children (mean age < 60 months) with ASD. Meta-analyses of three parent mental health outcomes (stress, depressive symptomatology, and general mental health) and seven parent–child relationship outcomes (parent responsiveness, affect, directiveness, non-directiveness, child initiation, infant positive affect, and infant attentiveness) were conducted using both mean and standardized mean differences. Results: Thirty-two RCTs met the inclusion criteria (2336 participants). Six RCTs showed improved maternal mental health (Cohen's d = 0.41–1.15), with moderate certainty of evidence for improvement in parental stress. There was low and high certainty of evidence of treatment effect on parental depressive symptoms and general mental health respectively, which did not show significant treatment effects post-intervention. Seventeen RCTs showed improved parent–child relationship (d = 0.33–2.28; low certainty of evidence). Interpretation: Results demonstrated that parenting interventions promoting responsiveness had the largest positive effect on the parent–child relationship and a moderate effect on overall parenting stress; however, the studies were heterogeneous, making it difficult to identify the intervention components that were responsible for the positive treatment effects. There was no significant treatment effect for studies targeting depression or general mental health, likely due to the lack of an interventional component directly targeting mental health.