Rationale:Small-cell undifferentiated carcinoma (SmCC), as an aggressive malignancy, are most commonly arising in lung. Extrapulmonary SmCC is rare. It was reported that SmCC accounts for only 0.1% to 0.2% of colorectal cancers. Currently, no standard treatment regimen is recommended. Here, we presented a case of SmCC from rectum. The patient achieved pathological complete response (pCR) after surgery, which makes us feel gratified, and we are also eager to share this successful case with more peers to provide more references for clinical decision-making.Patient concerns:A 32-year-old male patient presented himself to our outpatient clinic with defecation difficulty for more than 1 month in November 2021.Diagnoses:Colonoscopy revealed a rectal mass 4 cm from the anal margin. Pelvic magnetic resonance imaging revealed a mass in the rectal wall, consistent with the appearance of rectal carcinoma. Cancer cell was found after several biopsies and the immunohistochemistry indicated rectal SmCC.Interventions:Considering that the patient is very young and the malignancy of SmCC is very high, our treatment plan is also very cautious. Many literatures were also searched, but the literature on rectal SmCC is few and the prognosis is poor. Subsequently, we combined the treatment principles of rectal cancer and small cell lung cancer to develop an individualized treatment plan for patients. The patient received neoadjuvant chemoradiotherapy (nCRT) (short-course radiotherapy: 25 Gy/5 fractions, chemotherapy: etoposide + nedaplatin) combined with immune checkpoint inhibitors (ICIs) (tislelizumab). Then, the patient received laparoscopic radical transabdominal resection of rectal carcinoma with a temporary stoma on June 27, 2022.Outcomes:Postoperative pathology showed that there was chronic inflammation in the rectal mucosa without residual cancer, which meant that the patient achieved pCR after nCRT combined with ICIs. On August 15, 2024, the patient returned to our hospital for review, and no signs of recurrence and metastasis were found. By the time this article is submitted, the patient has survived for more than 35 months.Lessons:This is the first to be reported in a rectal SmCC patient who achieved pCR after nCRT combined with ICIs, which may provide supporting data for using this treatment option for rectal SmCC.