Mixed neuroendocrine–non-neuroendocrine neoplasms (MiNENs) are uncommon tumours with both adenocarcinoma and neuroendocrine carcinoma constituents. Their treatment is fraught with diagnostic difficulty and aggressive behaviour largely determined by the neuroendocrine component. Here, we report the case of a 44-year-old male patient diagnosed with a rectal MiNEN that consisted of moderately differentiated adenocarcinoma and high-grade neuroendocrine carcinoma. Regardless of neoadjuvant chemoradiotherapy, surgical resection revealed a mixed tumor. Follow-up liver metastases were proven to be purely high-grade neuroendocrine carcinoma by histopathology and immunohistochemistry. The present case highlights the rare phenomenon of selective neuroendocrine metastasis from a MiNEN primary tumor, underscoring the need for thorough histopathological assessment of metastatic sites and individualized management strategies targeting the most aggressive tumor component.