Background There are few readily available, relatively easy to use and culturally adaptable developmental assessment tools for young children in southern Africa. The overarching aim of this study is to test the psychometric properties, contextual appropriateness and cut-off scores across 21 age groups of the Ages and Stages Questionnaire Third Edition (Squires & Bricker, 2009) among a group of typically developing children in South Africa and Zambia through a combination of both caregiver-report and direct observations, and to compare children's performance across sociodemographic variables. Methods The sample consisted of 853 children (50.5% Zambia, with 50.1% girls for Zambia and 50% girls for South Africa) aged 2months to 60 months. Information on caregiver employment, education and household assets were also obtained. Results The psychometric properties of the ASQ-3 in southern Africa are consistent with those found in the extant literature. Analysis of item difficulty at each age reveals adequate levels of difficulty for majority of the items, with exception of the problem solving domain where half of the items at 54 and 60 months have poor pass rates. Sociodemographic variables were significantly associated with children's performance: higher caregiver levels of education are associated with higher toddler scores on the personal-social domain and higher preschooler scores on the problem solving domain; children whose caregivers earn a salary have higher fine motor scores during toddlerhood and higher problem solving scores during preschool and children who attend preschools have higher gross motor scores during toddlerhood and higher fine motor scores during the preschool years. Conclusions Findings provide evidence to support the psychometric properties and feasibility of using the ASQ-3 in both South Africa and Zambia through a combination of caregiver-report and direct observations.