Only 50% of Dutch children aged 0-4 years receive sufficient daily vitamin D supplementation. This study aims to investigate the effectiveness of implementation intentions in promoting vitamin D supplementation among young children. An electronic survey was conducted among parents of children aged 0-4 (n = 171). These parents were randomly assigned to two groups: one that received implementation intention instructions and one that did not. At follow-up, there were no significant between group differences in any outcome measures. These results suggest that merely asking parents to formulate an implementation intention with respect to giving their child daily vitamin D supplementation is insufficient to improve vitamin D intake among young children. However, testing the intervention via the Internet may not have allowed us to exploit the full potential of the strategy. Investigation of the use of implementation intentions in the setting of toddler consultation clinics is therefore recommended.