A 74-year-oldwoman hadconsultedthe department of urology in our hospital because of microscopic hematuria. Cystoscopy revealeda urinary bladder tumor, suspectedas an adenocarcinoma basedon biopsy. MRI showeda cystic tumor of the appendix with vesical fistula; therefore, she underwent an operation with a diagnosis of appendiceal cancer invading the urinary bladder. During the operation, we found that the appendix sunk into the urinary bladder with right adnexa. Therefore, we performed ileocecal resection, partial resection of the urinary bladder, and right adnexectomy. Macroscopically, the bladder was filled with a large number of mucus lumps. A papillary tumor, 4 cm in size, growing in the lumen of the bladder was detectedat the invasion site. Microscopically, proliferating carcinoma cells in a papillary form were observedin the lumen of the appendix with mucus production, invading the wall of the urinary bladder at the fundus of the appendix. Thus, the patient was diagnosed with mucinous adenocarcinoma of the appendix(V, type 1, 45×30 mm, muc, pT4b[SI, urinary bladder], int, INF c, ly0, v0, pN0, cM0, pStage II ). Primary appendiceal cancer invading the urinary bladder is very rare; herein, we report a rare case of appendiceal mucinous adenocarcinoma detected with a bladder tumor and present a literature review.