The authors report a case of myxoid adrenal cortical carcinoma (ACC) clinically manifesting as primary hyperaldosteronism. The 82-year-old female patient had a history of hypertension and was sent to the emergency room because of change in consciousness. Ventricular fibrillation occurred, and severe hypokalemia was found. Increased renal loss of potassium, high serum aldosterone level, low renin activity, and a huge tumor in the left suprarenal area were revealed when tests were conducted to determine the cause of her hypokalemia. Left adrenalectomy was performed. The tumor measured 13 cm in diameter and showed a heterogeneous cut surface with gelatinous material. Microscopically, the lesion was composed of polygonal cells with eosinophilic cytoplasm and arranged in arborizing cords in a myxoid background. Capsular and vascular invasion were observed. The tumor stained positive for synaptophysin, melan-A, vimentin, and α-inhibin but negative for cytokeratin. A primary myxoid ACC was diagnosed, which is a rare histological variant. The authors review 13 other reported cases. Most of these were functional tumors causing Cushing syndrome, and only 2 cases presented as primary hyperaldosteronism. All cases had similar microscopic and immunohistochemical features. Distal metastases and local recurrence were not uncommon. Close clinical follow-up is imperative.