Sepsis is a leading cause of neonatal morbidity and mortality worldwide, especially in low- and middle- income countries (LMIC). It is estimated to cause almost 1 million deaths that accounts for more than 25.0% of neonatal deaths worldwide. In Bangladesh the incidence of neonatal sepsis is 13.4% and sepsis contributes to almost 19.9% of all neonatal death. Vitamin D deficiency in mothers and neonates is being recognized increasingly as a leading cause of many adverse health effects in the newborn infant, including sepsis. Vitamin D plays an important role in calcium and phosphorus homeostasis as well as immunomodulatory actions on immune system. The aim of the study was to determine the association of serum 25(OH) vitamin D deficiency with late onset neonatal sepsis in term and late preterm neonates. This cross-sectional analytic study was conducted in the department of Neonatology, Mymensingh Medical College Hospital, Bangladesh from November 2021 to October 2022. Neonates who were admitted with suspected late onset neonatal sepsis were the study population. Neonates who visited during the same period due to physiological hyper-bilirubinemia (without sepsis) were enrolled as control group. Vitamin D deficiency was found in both sepsis group and control group. But in comparison the sepsis group had significantly lower mean±SD serum vitamin D levels 13.99±6.07 ng/ml than the control group 20.56±5.93 ng/ml (p=0.001). Similarly, vitamin deficiency was also found in mothers of both groups. But mothers of septic neonates also had significantly lower mean±SD vitamin D levels 16.36±6.25 ng/ml than the mothers of non-septic neonates 23.06±5.85 ng/ml (p=0.001). There was strong positive correlation of neonatal 25(OH) D with maternal 25(OH) D in both case (r =0.983; p<0.001) and control (r = 0.970; p<0.001).