Purpose <p>Vitamin D is a micronutrient that is necessary for bone health as well as the regulation of mineral homeostasis. This study aims to assess the vitamin D status and its determinants among infants and toddlers in Hong Kong, two of the most vulnerable groups at risk of vitamin D insufficiency.</p> Methods <p>A multi-centre cross-sectional study recruiting 887 infants and toddlers was conducted. A comprehensive assessment on potential factors that can influence vitamin D status as well as serum vitamin D concentration was conducted for all participants. Regression analyses were conducted to assess the factors contributing to the vitamin D insufficiency risk.</p> Results <p>124 (14.0%) infants and toddlers were vitamin D insufficient and 44 (5.0%) were vitamin D deficient, with higher prevalence observed among younger infants (aged 2 to 6 months). Majority did not consume any vitamin D-containing supplement (86.5%) which was associated with higher risk of vitamin D insufficiency (aOR = 2.16, <i>p</i> = 0.017). An increment of 100IU in total daily vitamin D intake lowered the risk of vitamin D insufficiency (aOR = 0.50, <i>p</i> &lt; 0.001) in older infants and toddlers. Being breastfed increased vitamin D insufficiency risk in younger infants (aOR = 24.91, <i>p</i> &lt; 0.001) and in older infants and toddlers (aOR = 7.36, <i>p</i> &lt; 0.001). Further analyses identified distinctive pattern among factors that can influence 25(OH)D3, 25(OH)D2, and 3-epi-25(OH)D3 concentrations.</p> Conclusion <p>Low total vitamin D intake and lack of supplementation practice were the key contributing factors to vitamin D insufficiency among infants and toddlers in Hong Kong. Detailed guidelines and support should be provided to meet their respective daily vitamin D intake requirement to further tackle the vitamin D insufficiency and deficiency among these vulnerable populations.</p>