Mixed adenoneuroendocrine carcinoma (MANEC) is a rare tumour of the gastrointestinal tract that consists of a dual adenocarcinomatous and neuroendocrine differentiation, each component representing at least 30% of the tumour. We report a case of a 68-year-old man who presented with two-month history of postprandial pain and vomiting. Gastric endoscopy revealed a polypoid mass in the lower part of the oesophagus. In contrast to the majority of these tumours, this biopsy was immunohistochemically positive for chromogranin A, and synaptophysin and Ki-67 index was 50% and the tumour was diagnosed as poorly differentiated neuroendocrine carcinoma of the oesophagus. The patient underwent surgery and lower oesophagus resection was performed. Based on the histopathology and immunohistochemistry of the tumour in the oesophagogastrectomy specimen, a mixed adenoneuroendocrine carcinoma (MANEC) was diagnosed. The objective of this case report is to advocate for the focus on the MANEC diagnosis as such patients need to be referred to a centre of excellence with expertise in NET tumours, to have the correct diagnostic work-up, treatment, and secondary diagnostic procedures performed at progression, as this will have paramount influence of the choice of treatment.