Background: With the report of high prevalence rate of Vitamin D deficiency in the literature since the 1980s, the condition can be considered an epidemic in Saudi Arabia. However, some specific steps have been taken by the Ministry of Health to tackle the problem. Moreover, little is known about Knowledge and Adherence of Vitamin D supplementation for infants. Women’s health is of particular concern due to reproduction which affects their health. The Saudi community has changed recently in regard to women’s rights and education. Those changes are expected to have a great impact on breastfeeding practice and contraception use, Knowledge and Adherence of Vitamin D supplementation for infants. Vitamin D is known to have essential roles in the human body. However, the case of vitamin deficiency is reported to increase in many infants worldwide, especially in Saudi Arabia. Mothers, having adequate knowledge about vitamin D and its supplementation, may lead to possible lowering of the cases of vitamin D deficiency and its complications. Many international medical organizations recommend vitamin D supplementation for infants, especially exclusively breastfed infants, however, data regarding the vitamin D status in infants are lacking. Such data would help to support physician decisions and guide medical practice Ami of the study: To Level of knowledge and adherence to Vitamin D Supplementation among Infants attending Well-Baby Clinic in Primary Health Care Center in Makkah, Saudi Arabia 2021. Methods: A cross-sectional study was conducted among (200) mothers of infants attending the well-baby clinic in the Primary Health clinic in 2021at Makkah AL-Mokarramah random sampling technique was applied and an Arabic self-administered questionnaire used for data collection. Result: Regarding the Adherence to Vitamin D Supplement the majority of participant answer No were(66.0%) While a significant relation while p=0.001 and X 2 19.845.regarding the Level of Vitamin D knowledge the most of participant weak were (65.0%) while p=0.001 and X 2 92.68. Conclusion: Most full-term, exclusively breastfed infants have serum vitamin D concentration below sufficiency level at 6 months of age. However, vitamin D supplementation (400 IU/day) improves their vitamin D status and prevents vitamin D deficiency.