Hydrogen sulfide (H2S) is a toxic gas produced via breakdown of organic matter. Hydrogen sulfide exposure can cause symptoms ranging in severity from mild effects (dizziness, headache, nausea) to severe lactic acidosis, respiratory failure, pulmonary edema, cardiac arrhythmias and death. Treatment modalities include oral countermeasures and 100% FiO2 with supportive therapy. However, case studies utilizing hyperbaric oxygen (HBO2) therapy have been reported with general benefit seen in severe cases of toxicity. In this report, cases of mild to moderate H2S toxicity occurred aboard a U.S. Navy ship, resulting in a mass casualty incident of more than 30 patients. Patient symptoms included dizziness, headaches, nausea, vomiting, and one patient with altered mental status. Most patients' symptoms resolved after several hours of supportive therapy, but six patients had symptoms refractory to 100% FiO2 at 1 atm. These six patients received HBO2 therapy with a USN Treatment Table 9 after consultation with the local emergency room and hyperbaric assets. Four separate chambers were utilized, including two chambers onboard USN ships and the local explosive ordnance disposal (EOD) chamber. Complete resolution of symptoms in all six patients was achieved within the first breathing period. Patients were monitored after treatment aboard the USN ship medical department. No patients required emergency department care. These cases demonstrate an expanded use of HBO2 to include moderate cases of H2S toxicity refractory to first-line therapy.