Background: Malnutrition manifests as undernutrition and overnutrition, with stunting, underweight, and wasting being critical forms of undernutrition that hinder the physical and cognitive development of children under five in Bangladesh. This study aims to assess the nutritional status of children under five and identify key determinants of undernutrition. Methods: This study utilized secondary data from the 2019 Multiple Indicator Cluster Survey (MICS), including 22,448 children under five. The analysis focused on socio-demographic factors. Chi-square and t-tests were used for hypothesis testing, and logistic regression identified determinants of undernutrition. Results: The prevalence of multiple forms of undernutrition was 3.2%, with stunting affecting 27.9% of children, underweight 22.6%, and wasting 9.8%. Among districts, stunting was most prevalent in Sunamganj (45.9%), underweight in Habiganj (37.9%), and wasting in Lakshmipur (15.9%). Children from the poorest households had significantly higher odds of experiencing stunting (aOR: 2.2, 95% CI: 1.92-2.51), underweight (aOR: 2.4, 95% CI: 2.04-2.73), and wasting (aOR: 1.5, 95% CI: 1.24-1.83) compared to those from the richest households. Additionally, children in families with a history of child mortality had increased odds of stunting (aOR: 1.2, 95% CI: 1.05-1.32), underweight (aOR: 1.2, 95% CI: 1.07-1.36), and wasting (aOR: 1.2, 95% CI: 1.06-1.45). Conclusions: Household wealth and a history of child mortality were key determinants of undernutrition among children under five. Parental education, particularly the lack of primary education in both parents, was strongly associated with stunting and overall undernutrition. These findings highlight the need to improve socio-economic conditions and promote parental education to reduce childhood undernutrition in Bangladesh.