Background Pneumonia is a critical global health concern that often results in severe complications and fatalities, especially among young children. Zinc plays a crucial role in immune function and maintaining respiratory epithelial integrity. Despite its importance, data on the prevalence of zinc deficiency and its impact on pneumonia severity in Vietnamese children are limited. Objectives This study aimed to investigate the prevalence of zinc deficiency and its association with pneumonia severity in Vietnamese children under five years old. The findings could significantly contribute to our understanding of the role of zinc in pneumonia severity, guiding future public health interventions, nutritional policies, and clinical practices to prevent zinc deficiency and reduce pneumonia morbidity and mortality in children. Methods An analytical cross-sectional study was conducted at a major pediatric center in Southwestern Vietnam from December 2022 to February 2024, involving 222 children aged 2 to 59 months diagnosed with pneumonia. Clinical assessments and laboratory measurements, including serum zinc levels, were performed. Statistical analyses were conducted to compare clinical characteristics and outcomes between zinc-deficient and non-deficient groups. Multivariable logistic regression was used to assess the association between zinc deficiency and pneumonia severity, with statistical significance set at p<0.05. Results The prevalence of zinc deficiency among children with pneumonia was 74.3%. Zinc-deficient children showed a significantly higher proportion of severe pneumonia (57.6% vs. 8.8%, p<0.001), as well as a higher proportion of high fever, poor feeding, vomiting, and respiratory distress compared to non-deficient children (p<0.001). Multivariable logistic regression identified zinc deficiency as an independent predictor of severe pneumonia (aOR=13.1, 95% CI: 4.7-36.8, p<0.001). Conclusion Zinc deficiency was prevalent among Vietnamese children with pneumonia and was associated with an increased risk of severe pneumonia.