Introduction: The nutritional status of children with Attention-Deficit/Hyperactivity Disorder (ADHD) and Down Syndrome (DS) is influenced by multiple food environment determinants. However, the precise relationship between food environment determinants and nutritional outcomes in these children remains unclear. This study aimed to understand the food environment determinants that are associated with the intake and nutritional status of these children in Malaysia, Lao PDR, and Indonesia. Methods: This cross-sectional study involved 7- to 18-year-old students with ADHD and DS in three Southeast Asian countries. Sociodemographic characteristics, dietary habits, and repeated 24-hour food recall data were collected through a structured questionnaire. Anthropometric data were obtained through direct measurements and analysed using the WHO AnthroPlus software. Further analyses were done using descriptive statistics and chi-square tests. Results: Total participants were 284 children, comprised of 37.7% with ADHD and 62.3% with DS; 62.3% of the children were boys and 37.7% were girls. Among the food environment determinants collected during the study, monthly household income and school food assistance status had significant associations (p<0.05) with BMI-for-age z-score category and total energy, protein, zinc, and calcium intakes. While the country, type of residence, and type of school had significant associations with total energy, protein, zinc, and calcium intakes. Conclusions: Food environment determinants, particularly household income and access to school food assistance, significantly influence the nutritional status and intake of children with ADHD and Down Syndrome. Thus, strengthening inclusive nutrition programmes may improve outcomes for this vulnerable group in Southeast Asia. © (2026), (Malaysian Journal of Nutrition). All rights reserved.