BACKGROUND: Pandemic-related weight gain disproportionately has affected children from socioeconomically disadvantaged backgrounds. However, key determinants of childhood obesity among these vulnerable children remain relatively underexplored. METHODS: In a secondary data analysis using a longitudinal dataset, the relationships between child weight status and its individual and environmental influences among vulnerable children were examined based on an ecological model. We analyzed three-point time-series data for 1308 participants from a Korean national panel survey of vulnerable children. Adiposity estimates were calculated using body mass index (BMI) RESULTS: The panel regression analysis revealed that 2 factors, namely "lack of adequate sleep time" at the individual level and "suboptimal relationships with childcare teachers" at the interpersonal level, had a significant impact on the BMI CONCLUSIONS: It is crucial to develop strategies that ensure sufficient sleep time and to actively involve childcare centers in implementing place-based interventions and initiatives to prevent obesity among vulnerable children. Moreover, it is essential to continue efforts in identifying the significant determinants of childhood obesity among vulnerable populations by considering the major environments in which these children live and grow.