Abstract Background Spirocercosis in dogs is characterized by esophageal nodules that can undergo neoplastic transformation. Hypovitaminosis D has been associated with neoplasia formation. We hypothesized hypovitaminosis D in neoplastic spirocercosis and that it could be a risk factor for neoplastic transformation. Objective To measure and compare vitamin D status, assessed by serum 25-hydroxyvitamin D [25(OH)D] concentrations in non-neoplastic (n = 25) and neoplastic (n = 26) spirocercosis client-owned dogs and healthy dogs (n = 24). Animals Twenty-five non-neoplastic dogs, 26 neoplastic dogs, and 24 healthy dogs. Methods Fifty-one dogs were randomly selected from 119 dogs diagnosed with spirocercosis presenting to our hospital, and further divided into non-neoplastic or neoplastic groups. Exclusion criteria included dogs less than 1 year old, with concurrent diseases, received corticosteroids, or treated prophylactically for spirocercosis. Serum 25(OH)D concentration was measured by high-performance liquid chromatography. Spirocercosis dogs' appetites were graded and compared. Results Serum 25(OH)D concentrations were significantly different among all groups (P < .001). 25-Hydroxyvitamin D concentrations were significantly lower in neoplastic group (median 30.7 nmol/L [range 14.7–62.2]) compared to non-neoplastic (median 52.7 nmol/L [range 19.1–129.7, P < .05]) and healthy groups (median 74.6 nmol/L [range 37.4–130.5, P < .005]). 25-hydroxyvitamin D concentrations were significantly lower in non-neoplastic spirocercosis dogs compared to healthy ones (P < .05). Neoplastic and non-neoplastic spirocercosis dogs had similar appetite scores (P = 1.0). 25-Hydroxyvitamin D concentrations were not significantly different between dogs with normal (P = .087) and abnormal (P = .125) appetites within neoplastic and non-neoplastic spirocercosis groups. Conclusions and Clinical Importance Further studies are warranted to determine potential use of vitamin D treatment in spirocercosis and explore role of hypovitaminosis D in pathogenesis of malignant transformation.