Descending necrotizing mediastinitis is a rare but serious complication of oropharyngeal and deep neck infection which spreads down to the mediastinum via the cervical-facial planes. Its mortality rate remains high even with aggressive surgical drainage and appropriate antibiotics. Here, a case of descending necrotizing mediastinitis secondary to peritonsillar abscess is reported. It was successfully treated with hyperbaric oxygen and antibiotics followed by surgical drainage. Based on this report, hyperbaric oxygen therapy might be of great value as an adjunctive management to control this fatal infection.