Objective This systematic review explored the effectiveness of internet-delivered interventions in improving psychological outcomes of informal caregivers for neurodegenerative-disorder (ND) patients. Methods We searched seven databases for English-language papers published from 1999 to May 2021. Study-eligibility required that interventions used a minimum 50% internet-facilitation, targeting unpaid, adult informal caregivers of community-based ND-patients. We included randomised controlled trials (RCTs) and pre-post evaluative studies reporting outcomes for at least one-time point post-intervention. Independent quality checks on abstract and full-text screening were completed. Data extraction encompassed interventions’ features, approaches, theoretical bases and delivery-modes. The Integrated quality Criteria for the Review Of Multiple Study designs (ICROMS) framework assessed risk of bias. Alongside narrative synthesis, we calculated meta-analyses on post-intervention using outcome measures from at least two RCTs to assess effectiveness. Results Searches yielded 51 eligible studies with 3180 participants. In 48 studies, caregivers supported a dementia-diagnosed individual. Intervention-durations encompassed four weeks to 12 months, with usage-frequency either prescribed or participant-determined. The most frequently-used approach was education, followed by social support. We calculated meta-analyses using data from 16 RCTs. Internet-delivered interventions were superior in improving mastery (g = 1.17 [95% CI; 0.1 to 2.24], p = 0.03) and reducing anxiety (g = -1.29 [95% CI; −1.56 to −1.01], p < 0.01), compared to all controls. Findings were equivocal for caregivers’ quality of life, burden and other outcomes. High heterogeneity reflected the multifarious combinations of approaches and delivery-modes, precluding assessment of the most efficacious intervention features. Analyses using burden and self-efficacy outcomes’ follow-up data were also non-significant compared to all comparator-types. Although 32 studies met the ICROMS threshold scores, we rated most studies’ evidence quality as ‘very-low’. Conclusions This review demonstrated some evidence for the efficacy of internet-delivered interventions targeting informal ND-caregivers. However, more rigorous studies, with longer follow-ups across outcomes and involving NDs other than dementia, are imperative to enhance the knowledge-base.