Metanephrine-secreting adrenocortical carcinoma has been rarely described in humans and, to our knowledge, has not been previously confirmed in dogs. A 12-year-old castrated male Maltese was referred for an incidentally detected left adrenal mass. The dog had persistent systemic hypertension (160-210 mmHg) and urinary normetanephrine and metanephrine-to-creatinine ratios above the reference range, findings suggestive of pheochromocytoma. Computed tomography revealed a homogeneously enhancing adrenal mass without overt vascular invasion. After preoperative phenoxybenzamine therapy, laparoscopic adrenalectomy was performed. Histopathology demonstrated adrenocortical carcinoma with vascular tumor emboli. Immunohistochemistry demonstrated diffuse Melan-A positivity and chromogranin A negativity, with synaptophysin immunoreactivity in corresponding tumor regions on serial sections, supporting adrenocortical origin with partial neuroendocrine differentiation. Postoperatively, urinary catecholamine metabolite concentrations normalized, and serial blood pressure monitoring at approximately monthly re-evaluations documented no recurrence of systemic hypertension without antihypertensive therapy over an 11-month postoperative follow-up period, supporting the adrenal cortical tumor as the source of the preoperative biochemical and hemodynamic abnormalities.