Parents of children with additional needs experience compromised physical and mental health and higher stress, and their ability to engage in meaningful occupations is impacted by their additional caregiving tasks. Online interventions targeting mental health, stress, and well-being for parents of children with additional needs have potential to increase supports alongside occupational therapy direct services. The aims of this review are to systematically identify and synthesise evidence of effectiveness of online interventions aimed at improving mental health, stress, and well-being of parents of children with additional needs. Eight databases were searched up to July 2024 to identify online interventions for parents of children with additional needs. Studies were appraised for methodological quality. Standardised effect sizes were calculated, and meta-analyses of randomised control trials (RCTs) were conducted on outcomes of mental health, well-being, and stress. There was no consumer or community involvement. Systematic screening identified 30 papers that met inclusion criteria. Most were RCTs or controlled clinical trials (CCTs) with 'moderate' quality ratings: Three were designed by occupational therapists. There was substantial variation in intervention types, methods of delivery, outcomes, and outcome measures used. The pooled standardised effect size estimates (ES) and the lower and upper confidence intervals (CI) of online interventions from RCTs post-interventions were significant for mental health ES = 0.47, 95% CI (0.18, 0.77), p = 0.002; stress ES = 1.27, 95% CI (0.56, 1.98), p = 0.000; and well-being ES = 0.65, 95% CI (1.2, 0.06), p = 0.03, respectively. The online interventions that aimed to improve mental health, stress, and well-being included in this meta-analysis were effective. Occupational therapists supporting families play an important role in guiding parents of children with additional needs to evidence-based interventions to support them with addressing their own mental health and well-being needs.