In a 64-year-old female patient, a submucosal elevation with a diameter of 0.9 cm was found incidentally in the anus, 15 cm from the anus. Endoscopic ultrasonography revealed a hypoechoic mass originating from the muscularis mucosae. Preoperative diagnosis based on intraoperative ultrasound was rectal neuroendocrine tumor. The lesion was completely removed by endoscopic submucosal excavation with negative resection margins, pathology confirmed primary rectal adenocarcinoma. After the operation, there was a recovery without complications. The patient was followed for 4 years and 11 months without tumor recurrence, metastases or damage to intestinal functions, with a good quality of life maintained. The case highlights the need for careful evaluation of smooth submucosal rectal lesions due to the risk of overlooking malignancy. Endoscopic submucosal excavation allowed complete resection with minimal trauma, rapid recovery, and favorable long-term safety.