A case of atypical mucosal xanthomas in a 14-year-old Shi-Tzu dog is reported. Grossly, 1-6-mm granular plaques or slightly elevated papillary nodules were found in the oral cavity (dorsal and ventral sides of the tongue, inner upper lip) and in the upper digestive tract (esophagus, stomach). Microscopically, subepithelial aggregates of large foamy cells were found in strong association with papillary epithelial hyperplasia. By immunohistochemistry, the majority of these cells tested positive for lysozyme and ACM1. The cells had cytoplasmic lipid content that stained positively with oil red O. These findings confirmed a monocyte/macrophage lineage. On the basis of macroscopic observations, microscopic changes, and the absence of a clinical metabolic defect, the condition in this dog appears similar to that of humans with oral verruciform xanthoma. The pathogenesis of these xanthomas in the dog remains obscure, although this condition appears to affect people and dogs with an Asian origin.