Congenital diaphragmatic hernia (CDH) is one of the most common major congenital anomalies. The presence of a hernia sac is associated with additional benefits in terms of improved neonatal survival. Here, we report a case of CDH with a hernia sac detected via postnatal ultrasonography. Our literature search did not find other cases where CDH with hernia sac was found by postnatal ultrasound in neonates. In prenatal imaging, the diagnosis of CDH with a hernia sac is challenging. In our case, the meniscus of the thymus was clearly noted, and smooth convexity between the hernia contents and thymus was detected. Although evaluation of the presence of a hernia sac with postnatal ultrasonography might be difficult, our findings suggest that a herniasac could be evaluated with postnatal ultrasonography.