Colorectal cancers are among the most common cancers worldwide, but neuroendocrine carcinomas (NECs) of the colorectum are rare, accounting for less than 1% of cases. This article describes the case of a 64-year-old man who developed an NEC at the site of a previously treated sigmoid adenocarcinoma. Following a sigmoid resection and adjuvant FOLFOX-based chemotherapy, the patient underwent regular surveillance. Two years after treatment, a secondary tumor lesion was detected. Histological and immunohistochemical analyses confirmed a well-differentiated grade 2 NEC. Management included local tumor resection, followed by multidisciplinary discussions regarding additional chemotherapy. This case highlights the challenges associated with the rare occurrence of NECs at sites previously treated for adenocarcinoma, with several hypotheses proposed to explain this phenomenon: tumor transformation, selection of tumor subclones by chemotherapy, undetected coexistence, or a distinct neoplastic process. Diagnosis relies on immunohistochemistry and advanced imaging, while treatment combines surgery, platinum based chemotherapy, and rigorous follow-up. This report underscores the importance of individualized surveillance to detect recurrences or new lesions early.