A 77‐year‐old man presented with abdominal pain. Abdominal computed tomography (CT) revealed a bladder tumor extending from the dome to the left lateral wall, with suspected muscle invasion and left hydronephrosis (Figure 1A), along with increased perirectal fat density, rectal wall thickening causing obstruction, and ascites (Figure 1B). No evidence of lymph node metastasis was observed. Because of the rectal obstruction, the patient was admitted and underwent colonoscopy followed by transanal decompression tube placement. Colonoscopy showed edematous mucosa at the site of rectal obstruction, and biopsy showed carcinoma with unclear mucosal architecture and differentiation, precluding identification of the primary site. Contrast‐enhanced CT on hospital day 3 revealed the development of right hydronephrosis, as well as rectal wall alternating hypo‐ and hyperenhancement, showing a malignant target sign (Figure 1C) [1]. Cystoscopy showed a patent right ureteral orifice and a partially necrotic mass predominantly involving the bladder dome (Figure 1D). Voided urine cytology was positive for malignancy. Serum carbohydrate antigen 125 (CA125), carbohydrate antigen 19–9 (CA19‐9), and Span‐1 levels were markedly elevated, whereas immunoglobulin G and immunoglobulin G4 levels were within normal range. Upper gastrointestinal endoscopy and abdominal ultrasonography revealed no evidence of gastric or pancreatic cancer. Urachal carcinoma, primary bladder adenocarcinoma, and urothelial carcinoma (UC) with glandular differentiation were considered in differential diagnosis. Transurethral resection of the bladder tumor (TUR‐BT), right ureteral stent placement, and laparoscopic sigmoid colostomy were performed on hospital day 6. Tumor invasion extending to the peritoneum was apparent (Figure 1E). No disseminated nodules were observed. Histopathological examination of the TUR‐BT specimen revealed solid and isolated invasive proliferation on a background of urothelium, with no variant histology identified (Figure 2A). No cystic components or glandular structures suggestive of urachal carcinoma or primary bladder adenocarcinoma were identified. Immunohistochemical staining showed positivity for cytokeratin 7, cytokeratin 20, GATA‐binding protein 3, and uroplakin II and negativity for caudal type homeobox 2, consistent with UC (Figure 2B–E). Cytology of ascites fluid and the rectal biopsy specimen demonstrated similar immunostaining patterns. Based on these findings, the patient was diagnosed with BC with peritoneal invasion, accompanied by malignant ascites and rectal involvement. Following the operation, the patient exhibited increased hepatobiliary enzyme levels, including aspartate aminotransferase, alanine aminotransferase, and gamma‐glutamyl transpeptidase (γ‐GTP), reaching a maximum of Common Terminology Criteria for Adverse Events (CTCAE) Grade 4. Contrast‐enhanced CT revealed obstructive cholangitis secondary to CBD obstruction, with no evidence of direct invasion or mass‐like lesions indicative of malignant retroperitoneal fibrosis. (Figure 3A). Endoscopic retrograde cholangiopancreatography demonstrated beak‐like narrowing of the bile duct, suggestive of external compression (Figure 3B), along with poor duodenal dilatation and edematous mucosa of the major duodenal papilla (Figure 3C). No endoscopic findings suggested cholangiocarcinoma, and bile duct cytology was negative. After biliary stenting, hepatobiliary enzyme abnormalities improved overall, with only γ‐GTP remaining mildly elevated at CTCAE Grade 2. The patient's Eastern Cooperative Oncology Group (ECOG) performance status (PS) was 1 at the initial presentation; however, it progressively deteriorated due to cancer progression and did not improve after biliary stenting. On postoperative day 44, at the time of initiation of systemic therapy, the ECOG PS had declined to 3, and the estimated glomerular filtration rate calculated using the Cockcroft–Gault formula was 24.4 mL/min. Based on these considerations, pembrolizumab monotherapy was initiated. Systemic therapy was ineffective, and the patient died of cancer on postoperative day 57.