Patients afflicted with melanoma show lower vitamin D serum levels (VDSL) than the healthy population. This hypothesis agrees with its well-known antiproliferative features. An observational study was carried out to collect VDSL in patients suffering from melanoma. Our aim was to identify a potential connection between low VDSL and the risk to incur melanoma. Furthermore, we studied the association between VDSL at the diagnosis of melanoma and other germane prognostic factors. The population held in regard was composed of 154 patients with a diagnosis of melanoma between 2016 and 2019. These patients were retrospectively collected from our follow-up storage. We compared VDSL to clinical and pathological parameters (age, sex, tumour location, Breslow's depth, Clark's level, histological subtype, ulceration, et aliqua). Moreover, we recruited a control group with negative melanoma history. Mean and median of VDSL were significantly lower in the melanoma group. Instead, we found a negative association between melanoma and VDSL > 30 ng/L (OR 0.11; p < 0.0001). No correlation between VDSL and both Breslow's depth and Clark's level was discovered, but the VDSL comparison between thin (depth ≤ 1 mm) and thick tumours (depth > 1 mm) revealed a statistically significant difference (21.1 ± 8.2 ng/L vs 17.8 ± 8.1; p = 0.01). Moreover, VDSL were significantly lower in melanomas with mitotic rate ≥ 1/mm2 (22.1 ± 8.3 ng/L; p < 0007). Nevertheless, no connection was found between VDSL and both ulceration and positive sentinel nodes (p = 0.76; p = 0.74). Besides, our study revealed no association between VDSL and histological subtype (p = 0.161). Lower VDSL correlate with thick and high mitotic rate tumours. Future prospective studies would investigate if appropriate upkeep of suitable VDSL can decrease the risk of primary and recurrent melanoma diagnosis.