Background: The relationship between primary open-angle glaucoma (POAG), a leading cause of permanent vision loss, and serum vitamin D levels is still the subject of ongoing research. This study aimed to analyze the connection between serum vitamin D levels and the severity of POAG. Methods: This case-control study was completed on 25 POAG patients and 25 control subjects. Based on changes in the MD of the visual field, the group of POAG patients was divided into mild, moderate, advanced, and severe groups. Every participant underwent a thorough history taking and an examination of their eyes. An enzyme-linked immunosorbent test was employed to establish serum vitamin D levels (ELISA). Results: The mean serum vitamin D level in the POAG group was 27.16 ng/mL (± 11.53), while that in the control group was 49.62 ng/mL (± 11.42). ROC analysis demonstrated that vitamin D was an excellent discriminator between the two groups (AUC = 0.926, 95% CI: 0.844–1.000). Vitamin D can predict severe POAG (AUC = 0.947, 95% CI: 0.858–1.000) with a ≤16.32 ng/mL cut-off value. Logistic regression analysis indicated that reduced vitamin D levels were a significant POAG predictor. Ordinal regression analysis showed that, in multivariable analysis, only vitamin D levels were a predictor for greater grades of POAG severity. Conclusion: The current study found a high correlation between serum vitamin D levels and POAG severity and that vitamin D deficiency was a reliable predictor of POAG occurrence.