Retroperitoneal bleeding is a rare and difficult to diagnose condition, defined as bleeding in the retroperitoneal space without associated trauma or iatrogenic manipulation. It has been associated with hematologic diseases and malignancies and is more common in patients receiving systemic anticoagulation. A 50-year-old man on aspirin presented with abdominal pain. Physical examination revealed abdominal tenderness and a palpable mass at the left abdominal area. An abdominal CT revealed a spontaneous retroperitoneal hematoma due to bleeding of an intraparenchymal branch of the left renal artery. The patient underwent left nephrectomy in order to control the bleeding. Pathology of the kidney showed evidence of acute and chronic microangiopathy, renal artery stenosis, and renal vein thrombosis. Further work-up led to diagnosis of primary antiphospholipid syndrome. Treatment of spontaneous retroperitoneal bleeding varies from conservative in hemodynamically stable patients to invasive or even surgery in hemodynamically unstable patients. In our case, open surgery was performed due to the rapidly deteriorating patient's condition and the inability to embolize the bleeding vessel by interventional radiology. Physicians should always think of retroperitoneal bleeding in patients presenting with abdominal pain and signs of hypovolemia, especially if they have a bleeding disorder or receive anticoagulants or antiplatelets.