Collision tumor of the appendix is an extremely rare entity, defined as the coexistence of two independent tumors located in the same site without transitional changes. We describe in this report the case of a 75-year-old man who presented with an acute abdominal pain, nausea and vomiting. Physical examination revealed an abdominal distension with a hypogastric mass. Abdominal computed tomography showed colonic obstruction related to a sigmoid tumor. Therefore, a total colectomy was made. Macroscopic specimen examination showed a sigmoid tumor associated to a cystic dilatation of the appendix tip with mucoid content. Histological examination of the appendix showed the co-existence of two independent tumors located in the tip, without transitional changes: pTis low-grade appendiceal mucinous neoplasia and grade 1 neuroendocrine tumor. The latter was discovered incidentally during histological examination. We draw attention through our presentation to the importance of a thorough macroscopic and histological examination of the appendix.