Objective: To investigate the expression level of interleukin-17D (IL-17D) in the serum of patients with severe pneumonia and its correlation with disease severity. Methods: This study included 50 patients with severe pneumonia who were diagnosed and treated in the hospital from May 2024 to May 2025. The expression level of IL-17D in the serum of all patients was recorded. Patients were divided into severe and mild groups based on their disease severity. Gender, age, disease duration, presence of fever, atelectasis, pneumothorax, interleukin-2 (IL-2), interleukin-4 (IL-4), interleukin-6 (IL-6), and interleukin-17D were selected as independent variables. Statistical software SPSS 22.00 was used for univariate analysis, and variables with statistical significance in the univariate analysis were included in a multivariate logistic regression analysis to determine the correlation between IL-17D and the severity of severe pneumonia. Results: The results of this study showed that the level of IL-17D in patients with severe pneumonia was significantly higher than the normal threshold. Univariate analysis indicated that atelectasis, IL-2, IL-6, and IL-17D were statistically significant (P < 0.05) and could be considered as influencing factors for the severity of severe pneumonia. Multivariate logistic regression analysis revealed that atelectasis (OR=2.141, 95% CI: 1.684–2.391), IL-2 (OR=2.884, 95% CI: 2.240–3.614), IL-6 (OR=2.571, 95% CI: 2.190–2.943), and IL-17D (OR=2.416, 95% CI: 2.093–2.735) were positively correlated with the severity of severe pneumonia. Conclusion: The expression level of IL-17D in the serum of patients with severe pneumonia is higher than the normal threshold and is positively correlated with disease severity.