Descending Necrotizing Mediastinitis (DNM) is uncommon infection of mediastinum arising from neck or oral cavity. Despite improvements in diagnostic imaging and treatment, its mortality rate remains relatively high. We describe a unique case of descending necrotizing mediastinitis secondary to the third branchial sinus. The patient was successfully treated with antibiotics and transcervical surgical drainage. The tract was removed in second staged procedure after that descending necrotizing mediastinitis was cured.