Evidence is needed on effective approaches to build parents' ability to promote child development feasible in low- and middle-income countries. Our objective was to synthesize impact of the Reach Up early childhood parenting program in several low- and middle-income countries and examine moderation by family and implementation characteristics. Systematic search using PubMed and Academic Search Elite/EBSCO Host. Randomized controlled trials of the Reach Up program from 1985 to February 2022 were selected. Data were extracted by 2 independent researchers. Primary outcomes were child cognitive, language, and motor development. Secondary outcomes were home stimulation and maternal depressive symptoms. We synthesized pooled effect sizes using random effect inverse-variance weighting and effect modification by testing pooled subgroup effect estimates using the χ2 test for heterogeneity. Average effect size across 18 studies ranged from 0.49 (95% confidence interval [CI] 0.32 to 0.66) for cognition, 0.38 (CI 0.24 to 0.51) for language, 0.27 (CI 0.13 to 0.40) for motor development, 0.37 (CI 0.21 to 0.54) for home stimulation, and -0.09 (CI -0.19 to 0.01) for maternal depressive symptoms. Impacts were larger in studies targeted to undernourished children, with mean enrollment older than age 12 months and intervention duration 6 to 12 months. Quality of evidence assessed with the Cochrane Assessment of Risk of Bias and GRADE system was moderate. Instruments used to assess child development varied. In moderator analyses, some subgroups included few studies. Reach Up benefits child development and home stimulation and is adaptable across cultures and delivery methods. Child and implementation characteristics modified the effects, with implications for scaling. [Abstract copyright: Copyright © 2023 by the American Academy of Pediatrics.]