Purpose <p>The purpose of this study was to examine the relationship between serum 25(OH)D levels and olfactory function in adults, given the emerging recognition of vitamin D as a neurosteroid with potential influence on sensory pathways. Specifically, the study aimed to determine whether vitamin D status is associated with olfactory threshold and identification performance, to explore the presence of a biologically plausible dose–response pattern, and to assess the predictive value of serum vitamin D levels for olfactory dysfunction.</p> Methods <p>In this cross-sectional study, 161 adults were enrolled, including 98 with vitamin D deficiency and 63 with sufficient levels. Serum 25(OH)D was measured by HPLC and categorized into four groups (&lt; 10, 10–20, 20–30, ≥ 30 ng/mL). Olfactory threshold and identification were assessed using the Sniffin’ Sticks rapid screening kit. Group comparisons, subgroup analyses, correlation tests, multiple linear regression, and ROC analysis (threshold &lt; 7) were performed.</p> Results <p>Vitamin D–deficient participants had significantly lower threshold and identification scores than controls (<i>p</i> = 0.008 and <i>p</i> &lt; 0.001). Both scores increased stepwise across vitamin D subgroups (<i>p</i> &lt; 0.001). Serum 25(OH)D correlated positively with threshold (<i>r</i> = 0.302) and identification (<i>r</i> = 0.432) (both <i>p</i> &lt; 0.001); in the deficiency group, the correlation persisted only for threshold. Vitamin D was an independent predictor of identification (β = 0.041; <i>p</i> &lt; 0.001). ROC analysis indicated that severe vitamin D deficiency (cutoff ≈ 10 ng/mL) was strongly associated with threshold-level olfactory impairment (AUC 0.636; specificity 95%).</p> Conclusion <p>Serum vitamin D level is associated with olfactory function, particularly odor identification, and shows a biologically plausible dose–response pattern. Vitamin D appears to be a specific risk indicator rather than a standalone diagnostic marker for olfactory dysfunction.</p>