Background: Caring for people with dementia is perceived as one of the most stressful and difficult forms of caring. Family caregivers always experience high levels of psychological burden and physical strain, so effective and practical support is essential. Internet-based supportive interventions can provide convenient and efficient support and education to potentially reduce the physical and psychological burden associated with providing care. Objective: This review aimed to (1) assess the efficacy of internet-based supportive interventions in ameliorating health outcomes for family caregivers of people with dementia, and (2) evaluate the potential effects of internet-based supportive intervention access by caregivers on their care recipients. Methods: An electronic literature search of the PubMed, EMBASE, Web of Science, CINAHL, Cochrane Library, and PsycINFO databases was conducted up to January 2020. Two reviewers (ML and YZ) worked independently to identify randomized controlled trials (RCTs) that met the inclusion criteria and independently extracted data. The quality of the included RCTs was evaluated using the approach recommended by the Cochrane Handbook for Systematic Reviews of Interventions. Standardized mean differences (SMDs) with 95% CIs were applied to calculate the pooled effect sizes. Results: In total, 17 RCTs met the eligibility criteria and were included in this systematic review. The meta-analysis showed that internet-based supportive interventions significantly ameliorated depressive symptoms (SMD=-0.21; 95% CI -0.31 to -0.10; P<.001), perceived stress (SMD=-0.40; 95% CI -0.55 to -0.24; P<.001), anxiety (SMD=-0.33; 95% CI -0.51 to -0.16; P<.001), and self-efficacy (SMD=0.19; 95% CI 0.05-0.33; P=.007) in dementia caregivers. No significant improvements were found in caregiver burden, coping competence, caregiver reactions to behavioral symptoms, or quality of life. Six studies assessed the unintended effects of internet-based supportive intervention access by caregivers on their care recipients. The results showed that internet-based supportive interventions had potential benefits on the quality of life and neuropsychiatric symptoms in care recipients. Conclusions: Internet-based supportive interventions are generally effective at ameliorating depressive symptoms, perceived stress, anxiety, and self-efficacy in dementia caregivers and have potential benefits on care recipients. Future studies are encouraged to adopt personalized internet-based supportive interventions to improve the health of family caregivers and their care recipients. Trial Registration: PROSPERO CRD42020162434; https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=162434.