Postnatal urinary ascites is a rare occurrence compared with other causes of peritoneal effusion at this age and its frequency in prenatal diagnosis. Spontaneous rupture of the bladder was diagnosed in a premature male infant presenting with postnatal ascites and anuric renal insufficiency. Surgical repair was required. No predisposing factor was associated with this rare complication. The urinary origin of the ascites was diagnosed based on the biochemical composition, allowing us to understand its mechanism and to treat it.