Background: Early childhood interventions have potential to offset the negative impact of early adversity. We evaluated the impact of a community-based parenting group intervention on child development in Zambia. Methods and Findings: We conducted a non-masked cluster-randomized controlled trial in Southern Province, Zambia. Thirty clusters of villages were matched based on population density and distance from the nearest health center, and randomly assigned to intervention (15 clusters, 268 caregiver-child dyads) or control (15 clusters, 258 caregiver-child dyads). Caregivers were eligible if they had a child six to 12 months old at baseline. In intervention clusters, caregivers were visited twice per month during the first year of the study by Child Development Agents (CDAs) and were invited to attend fortnightly parenting group meetings. Parenting groups selected 'head mothers' from their communities who were trained by CDAs to facilitate meetings and deliver a diverse parenting curriculum. The parenting group intervention, originally designed to run for one year, was extended and households were visited for a follow-up assessment at the end of year two. The control group did not receive any intervention. Intention-to-treat analysis was performed for primary outcomes measured at the year two follow-up: stunting and five domains of neurocognitive development measured using the Bayley Scale for Infant and Toddler Development (BSID-III). In order to show Cohen's d estimates, BSID-III composite scores were converted to z-scores by standardizing within the study population. 195/268 children (73%) in the intervention and 182/258 children (71%) in the control group were assessed at endline after two years. The intervention significantly reduced stunting (56/195 vs. 72/182; adjusted OR 0.45, 95% CI 0.22 to 0.92; p=0.028) and had a significant positive impact on language (β 0.14, 95% CI 0.01 to 0.27; p=0.039). The intervention did not significantly impact cognition (β 0.11, 95% CI -0.06 to 0.29; p=0.196), motor (β -0.01, 95% CI -0.25 to 0.24; p=0.964), adaptive behavior (β 0.21, 95% CI -0.03 to 0.44; p=0.088), or social-emotional development (β 0.20, 95% CI -0.04 to 0.44; p=0.098). Observed impacts may have been due in part to home visits by CDAs during the first year of the intervention. Conclusions: The results of this trial suggest that parenting groups hold promise for improving child development, particularly physical growth, in low-resource settings like Zambia. This study is registered at ClinicalTrials.gov (NCT02234726).