Primary aortoenteric fistula (PAEF) is a rare but catastrophic cause of massive gastrointestinal bleeding. Diagnosis of PAEF is difficult to make and is frequently delayed without strong clinical suspicion. Timely surgical intervention is essential for patient's survival. We report on a case of an 86-year-old woman with no history of abdominal surgery, who presented with abdominal pain. Initially, computed tomography scan showed an intra-abdominal abscess, located anterior to the aortic bifurcation. However, she was discharged without treatment because of spontaneous improvement on a follow-up computed tomography scan, which showed a newly developed right common iliac artery aneurysm. One week later, she was readmitted due to recurrent abdominal pain. On the second day of admission, sudden onset of gastrointestinal bleeding occurred for the first time. After several endoscopic examinations, an aortoenteric fistula bleeding site was found in the sigmoid colon, and aortography showed progression of a right common iliac artery aneurysm. We finally concluded that intra-abdominal abscess induced an infected aortic aneurysm and enteric fistula to the sigmoid colon. This case demonstrated an extremely rare type of PAEF to the sigmoid colon caused by an infected abdominal aortic aneurysm, which has rarely been reported.