The South East Asian Nutrition Surveys (SEANUTS) were conducted in 2010/2011 in Indonesia, Malaysia, Thailand and Vietnam in country representative samples totalling 16,744 children aged 0.5 to 12 years. Information on socio-demographic and behavioural variables was collected using questionnaires and anthropometric variables were measured. In a sub-sample of 2016 children, serum 25-hydroxy-vitamin D (25(OH)D) was determined. Data were analysed using SPSS complex sample with weight factors to report population representative data. Children were categorized as deficient (<25 nmol/L), insufficient (<50 nmol/L), inadequate (<75 nmol/L) or desirable (>=75 nmol/L). In Malaysia and Thailand, urban children had lower 25(OH)D than rural children. In all countries, except Vietnam, boys had higher 25(OH)D levels and older children had lower 25(OH)D. Regional differences after correcting for age, sex and area of residence were seen in all countries. In Thailand and Malaysia, 25(OH)D status was associated with religion. The percentage of children with adequate 25(OH)D (>=75 nmol/L) ranged from as low as 5% (Indonesia) to 20% (Vietnam). Vitamin D insufficiency (<50 nmol/L) was noted in 40 to 50% of children in all countries. Logistic regression showed that girls, urban area, region within the country and religion significantly increased the odds for being vitamin D insufficient. The high prevalence of vitamin D insufficiency in the (sub) tropical SEANUTS countries suggests a need for tailored approach to successfully combat this problem. Promoting active outdoor livestyle with safe sunlight exposure along with food-based strategies to improve vitamin D intake can be feasible options.