Aeromonas is well-recognized for causing diarrhea and post-traumatic wound infections. The most common Aeromonas species include Aeromonas hydrophila, Aeromonas caviae, and Aeromonas sobria. In cases of immunocompromise and malignancy, Aeromonas infections can prove fatal. Instances of deadly necrotizing fasciitis in the extremities due to Aeromonas infection have been documented. Herein, a case of previously unreported fatal retroperitoneal necrotizing fasciitis involving Aeromonas caviae in a patient with a history of gastric cancer is presented.