We report a case of an occult synchronous adrenal metastasis of rectal cancer. A left adrenal mass was found by computed tomography (CT) 6 months after low anterior resection for rectal cancer. This lesion was growing during adjuvant systemic chemotherapy. The level of serum carcinoembryonic antigen and that of adrenal hormones were within the normal range. Radical left adrenalectomy was performed, and histopathological examination revealed metastatic adenocarcinoma compatible with rectal cancer. Adrenal metastasis is not rare in autopsy series. The possibility of adrenal metastasis should be considered in the follow-up of rectal cancer patients. Surgery may be the treatment of choice in occult synchronous metastasis of adrenal gland in rectal cancer.