Background: Patients with necrotizing soft tissue infections (NSTIs) require prompt surgical debridement, appropriate intravenous antibiotics, and intensive support. Despite aggressive treatment, their mortality and morbidity rates remain high. The benefit of hyperbaric oxygen (HBO) as an adjunctive treatment is controversial. We investigated the effect of HBO in treating NSTIs. Methods: We analyzed clinical data retrospectively for 78 patients with NSTIs: 30 patients at one center were treated with surgery, antibiotics, and supportive care; 48 patients at a different center received adjunctive HBO treatment. We compared the two groups in terms of demographic characteristics, risk factors, NSTI microbiology, and patient outcomes. To identify variables associated with higher mortality rates, we used logistic regression analysis. Results: Demographic characteristics and risk factors were similar in the HBO and non-HBO groups. The mean patient age was 49.5 years; 37% of the patients were female, and 49% had diabetes mellitus. Patients underwent a mean of 3.0 excisional debridements. The median hospital length of stay was 16.5 days; the median duration of antibiotic use was 15.0 days. In 36% of patients, cultures were polymicrobial; group A Streptococcus was the organism most commonly isolated (28%). We identified no statistically significant differences in outcomes between the two groups. The mortality rate for the HBO group (8.3%) was lower, although not significantly different (p = 0.48), than that observed for the non-HBO group (13.3%). The number of debridements was greater in the HBO group (3.0; p = 0.03). The hospital length of stay and duration of antibiotic use were similar for the two groups. Multivariable analysis showed that hypotension on admission and immunosuppression were significant independent risk factors for death. Conclusions: Adjunctive use of HBO to treat NSTIs did not reduce the mortality rate, number of debridements, hospital length of stay, or duration of antibiotic use. Immunosupression and early hypotension were important risk factors associated with higher mortality rates in patients with NSTIs.