Carcinomas arising from embryonic adrenal rests are rare with only a handful of reported cases. We report a case of an adrenocortical carcinoma arising from an adrenal rest located between the bladder and prostate in a 51 year-old man. The patient presented following a year of rectal pain and constipation. Computed tomography (CT) scan revealed a 9 cm pelvic mass that appeared to arise from the soft tissue between the bladder and prostate, with displacement of the organs and narrowing of the rectal lumen, suspected to be a sarcoma. The surgically resected specimen showed a well-circumscribed, partially encapsulated tumor measuring 10.0 cm in greatest dimension. Both adrenal glands were identified intraoperatively. Grossly, the lesion was heterogeneous tan-brown to yellow, hemorrhagic and necrotic. Histology revealed sheets and nests of high-grade pleomorphic tumor cells with abundant clear to vacuolated cytoplasm with areas of necrosis, a high mitotic index (>10 mitoses/10 HPF) and foci suspicious for lymphovascular invasion. Adjacent adrenal cortical-type tissue was identified. Immunohistochemical stains revealed the tumor cells were weakly and focally positive for MiTF, Melan-A, inhibin and synaptophysin, and negative for CKAE1/AE3, HMB-45, calretinin, EMA, SMA, chromogranin, PAX8, MDM2 and CDK4. Based upon the morphologic and immunohistochemical profile, this was diagnosed as an adrenocortical carcinoma, arising in an adrenal rest. To our knowledge, no such tumor has been previously described in this location.