A rare congenital condition known as fetal megacystis, characterized by an unusual enlargement of the fetal urinary bladder, exhibits a distinctive keyhole appearance. This anomaly was identified during a routine prenatal ultrasound of a male fetus at 21 weeks gestational age. The degree of fetal megacystis can vary, often exceeding the normal bladder measurement of 7–8 mm during the first trimester. The atypical configuration of the bladder and urethra, leading to a narrowed and elongated shape, imparts the keyhole appearance to the bladder. While the exact etiology remains uncertain, it is believed to be associated with various factors, including genetic disorders and urinary tract obstruction. This condition can profoundly affect the health and development of the affected fetus, potentially resulting in renal dysfunction, urinary tract infections, and organ damage. Other congenital anomalies, such as those involving the kidneys, lungs, or skeletal system, may also be linked to this condition. The management and treatment approach is contingent upon several factors, including the severity of the condition, the presence of associated anomalies, and the patient's gestational age. Close monitoring through regular ultrasound examinations may be recommended in milder cases to track the condition's progression. Interventions such as fetal urinary bladder catheter implantation or neurosurgical procedures may be viable options in more severe instances.