The adenocarcinomas of the appendix are rare tumors of the gastrointestinal truct that often reveal no clear clinical symptoms and are not diagnosed in time. In this study, we analyze a rare case of an incidentally discovered adenocarcinoma of the appendix after appendicectomy. Case presentation: A Greek male patient of 37 years old patient presented to our hospital with signs of acute appendicitis and peritonitis. He underwent appendicectomy as therapeutic treatment. Post operational histological examination revealed in situ carcinoma of the appendix without high hematological tumor markers or visible metastasis in CT scan analysis. We decided to proceed to right hemicolectomy. The patient was discharged without complications and without displaying other symptoms in a monitoring interval of two months after surgery. Conclusion: Adenocarcinomas of the appendix are generally difficult to detect in the early stage leading to significant morbidity and mortality rates. Therefore, it is necessary to execute a very careful histopathological examination and perform a detailed intraperitoneal cleaning during operation, so that any indications of appendix adenocarcinomas in cases of appendectomy won’t be missed or misjudged.