The development of a foreign body granuloma from non-absorbable suture is a relatively uncommon condition, exceptional in the retroperitoneum of patients operated on for colorectal surgery. We report the case of a patient with a history of sigmoid neoplasm showing a retroperitoneal nodule on CT and high activity on PET, mimicking a tumor implant. Pathology found a foreign body granuloma. Foreign body granuloma from suture is a condition that may look identical to tumor relapse or metastatic disease on CT and PET scans, thus requiring pathology confirmation.