An adult female Northern red-bellied cooter (Pseudemys rubriventris) was presented with a 15-day history of anorexia and lethargy. On physical examination, the animal was considered to be in critical condition with a prominent swelling of the proximal aspect of the left forelimb. Laboratory analysis highlighted anemia, hyperuricemia, hyperuremia, low total protein, and elevated aspartate aminotransferase and lactate dehydrogenase. Radiographic images revealed a soft tissue mass infiltrating the humerus with boney involvement. Fine-needle aspiration was performed, and cytological evaluation indicated the sample was characterized by a prevalence of spindle to stellate cells frequently containing intracytoplasmatic brown to black granules associated with a moderate amount of mucin. The terrapin died 24 hours later with only the affected forelimb being submitted for histopathological evaluation. On gross examination, the proximal humerus was encircled by a 3-cm diameter, black, gelatinous mass with a mucoid, black appearance on cut surface. The histologic description of the mass was that it was a poorly demarcated, sparsely cellular, nonencapsulated neoplasm expanding from the dermis and infiltrating into the underlying soft tissues and bone. The neoplasm was composed of stellate cells embedded in an abundant periodic acid-Schiff–positive myxoid stroma. The cells frequently contained intracytoplasmic brown-black granules. These findings are consistent with a mucinous type of melanophoroma. Melanophoromas are tumors of melanin-producing cells and are well known in reptiles, most commonly in snakes and bearded dragons (Pogona vitticeps) but rarely reported in chelonians. The mucinous variant has only been described in the bearded dragon.