Primary Adenocarcinomas of the appendix are extremely rare tumor. We report a case of primary mucinous adenocarcinoma in a 40 year old lady misdiagnosed as having acute appendicitis. All the routine investigations were within normal limit. USG of abdomen showed dilated appendix with little fluid collection adjacent to it and no other abnormality was seen which suggested acute appendicitis. Appendicectomy was done and excised appendix was sent for histopathological examination. Mucinous Adenocarcinoma of the appendix was confirmed after histopathological examination. Right hemicolectomy was done as a second stage procedure. As some cases are incidentally discovered, this case emphasizes that histological examination of all appendicectomy specimens is mandatory.