BACKGROUND: Early childhood development (ECD) is a key determinant of long-term health, education, and wellbeing. However, one major global challenge is the lack of ECD assessment tools validated for use in low- and middle-income countries. To address this gap, the World Health Organization (WHO) launched in 2023 the Global Scales for Early Development, an open-access tool designed to generate reliable, valid, and internationally-comparable data on ECD for children aged 0-3 years globally. In this study, we examined the concurrent and convergent validity of the Global Scales for Early Development-long form (GSED-LF) for use with children aged 0-24 months in Kenya. METHODS: We analyzed baseline data collected in October-November 2023 as part of a cluster-randomized controlled trial evaluating a parenting program for improving ECD in rural Western Kenya. Primary caregivers (91% mothers) with a child under 24 months were enrolled across 64 villages in Busia and Homabay counties. The GSED-LF was administered to all children (N = 647). In a randomly selected sub-sample of children (N = 116), the Bayley Scales of Infant and Toddler Development (Bayley-III) and the Caregiver Reported Early Development Instruments (CREDI) were also administered to compare their scores with those from the GSED-LF. Concurrent validity of GSED-LF was assessed in terms of its correlations with Bayley-III and CREDI. Convergent validity of GSED-LF was examined with respect to parenting outcomes, including parental stimulation, home caregiving environment, and maternal mental health. RESULTS: GSED-LF scores had moderate associations with those on the Bayley and CREDI across the domains of cognitive, language, and motor development. GSED-LF had small associations with socioemotional development and relatively weaker concurrent validity for younger children under 12 months. GSED-LF also demonstrated good convergent validity in terms of showing moderate associations with maternal and paternal stimulation and the home caregiving environment. CONCLUSIONS: Overall, this study demonstrated the feasibility and initial validity of the GSED-LF as a direct assessment tool for use in rural Western Kenya. Additional psychometric analyses across diverse settings are needed to strengthen the reliability and validity evidence of the GSED-LF and establish it as a robust, globallyapplicable tool for assessing ECD in resource-limited settings.