Parent stress and mental health problems negatively impact early child development. This study aimed to systematically review and meta-analyze the effect of eHealth interventions on parent stress and mental health outcomes, and identify family- and program-level factors that may moderate treatment effects. A search of PsycINFO, Medline, CINAHL, Cochrane and Embase databases was conducted from their inception dates to July 2020. English-language controlled and open trials were included if they reported: (a) administration of an eHealth intervention, and (b) stress or mental health outcomes such as self-report or clinical diagnosis of anxiety and depression, among (c) parents of children who were aged 1-5 years old. Non-human studies, case reports, reviews, editorials, letters, dissertations, and books were excluded. Risk of bias was assessed using the National Institutes of Health (NIH) Study Quality Assessment Tools. Random-effects meta-analyses of standardized mean differences (SMD) were conducted and meta-regressions tested potential moderators. 38 studies were included (N = 4360 parents), from 13 countries (47.4% USA). Meta-analyses indicated eHealth interventions were associated with better self-reported mental health among parents (overall SMD = .368, 95% CI 0.228, 0.509), regardless of study design (k = 30 controlled, k = 8 pre-post) and across most outcomes (k = 17 anxiety, k = 19 depression, k = 12 parenting stress), with small to medium effect sizes. No significant family- or program-level moderators emerged. Despite different types and targets, eHealth interventions offer a promising and accessible option to promote mental health among parents of young children. Further research is needed on moderators and the long-term outcomes of eHealth interventions.