Primary bladder adenocarcinoma (PBA), especially the mucinous subtype, is a rare cancer that represents less than 2% of all bladder malignancies. Overlapping histopathological and immunohistochemical (IHC) features of PBA with metastatic colonic adenocarcinomas (MCA) make the final diagnosis very hard. We presented a 75-year-old woman presenting with hematuria and severe anaemia in the last two weeks. The abdominal computed tomography scan showed a tumor-sized 2x2 cm right to the bladder dome. The patient underwent partial cystectomy without postoperative complication. The histopathologic and IHC showed mucinous adenocarcinoma and could not distinguish between the PBA from MCA. Investigations to exclude MCA revealed no other primary malignant site and suggested PBA. In conclusion, mucinous PBA requires ruling out any possibility of a metastatic lesion that could arise from other organs. Treatment should be considered an individual approach based on the tumour location and size, the patient´s age, general condition, and comorbidities.