BACKGROUND Non-neurogenic neurogenic bladder involves fluctuating flow rates due to involuntary muscle contractions during voiding in those with normal neurological function. The diagnostic challenge lies in distinguishing between massive urinary bladder distension and ovarian tumors. While various pathologies mimicking ovarian tumors are documented, cases of a massively distended urinary bladder, known as giant urinary bladder, posing as such are notably scarce. CASE REPORT This case report presents the unique clinical scenario of a 31-year-old woman with Down syndrome who was initially misdiagnosed with an ovarian tumor due to progressive abdominal distention, reduced appetite, and weight loss. On presentation, she appeared dehydrated, with an abnormal renal profile. Despite hydration, the renal profile worsened. Initial ultrasound showed a large, uniloculated cystic lesion measuring 11×15 cm in the pelvis. Due to the size of the cyst, which appeared to be ovarian in origin, ovarian tumor was suspected. However, tumor markers were normal. A computed tomography scan subsequently showed a massively distended urinary bladder measuring 11.6×13.6×17.6 cm causing bilateral obstructive uropathy, with moderate hydronephrosis and hydroureter. Needing intermittent catheterization at first, the patient subsequently passed urine on her own following behavioral modification. CONCLUSIONS This rare case of non-neurogenic neurogenic bladder causing a giant urinary bladder in a patient with Down syndrome highlights the importance of an awareness of this condition for effective assessment and patient treatment.