IntroductionUrachus carcinoma is a rare malignancy with an aggressive potential and a poor prognosis, and evidence is limited for its diagnosis and treatment.Case presentationA 75‐year‐old man underwent fluorodeoxyglucose positron emission tomography/computed tomography for staging prostate cancer, and a mass (standardized uptake value max 9.5) was observed on the outside of the urinary bladder dome. T2‐weighted magnetic resonance imaging showed the urachus and a low‐intensity tumor, which suggested a malignant tumor. We suspected urachal carcinoma and performed total resection of the urachus and partial cystectomy. Pathological examination revealed mucosa‐associated lymphoid tissue lymphoma with cells positive for CD20 and negative for CD3, CD5, and cyclin D1. After the surgery, no recurrence has been observed for more than 2 years.ConclusionWe encountered an extremely rare case of mucosa‐associated lymphoid tissue lymphoma of the urachus. Surgical resection of the tumor provided an accurate diagnosis and good disease control.