Descending mediastinal abscesses arising from oral (dental or peritonsillar abscess) or deep neck infections (DNIs) often become fatal without surgical drainage. Odontogenic origin was the most common cause of DNIs. We report the first clinical case of a patient with descending mediastinal abscess caused by DNI from Eggerthia catenaformis a Gram-positive anaerobic rod that can be rarely a human pathogen. The patient underwent the extraction of carious molars, cervical and thoracic surgical drainage, and targeted antibiotic therapy with resolution of the infection.