The potential etiology of idiopathic sudden sensorineural hearing loss (ISSNHL) is cochlear ischemia, therefore, hyperbaric oxygen (HBO) therapy is a promising treatment, particularly in patients with severe hearing loss (≥70 dB). To evaluate the efficacy of HBO therapy. The medical records of patients diagnosed with ISSNHL were retrospectively reviewed (≥70 dB). Patients received HBO therapy 14 times in addition to systemic and intratympanic steroid therapy (HBO group), or systemic and intratympanic steroid therapy only (control group). Data from a total of 82 patients (83 ears) were included in the analysis; 37 (38 ears) in the HBO group and 45 (45 ears) in the control group. After 2 weeks’ treatment, hearing was significantly improved in the HBO group versus controls (weighted four-frequency average 28.1 ± 26.9 dB versus 14.8 ± 13.5 dB, respectively;  < .05), particularly in the low frequency groups (0.5 kHz, 1 kHz, 2 kHz;  < .05). These data demonstrate that HBO therapy is an effective initial treatment option for patients with ISSNHL suffering from severe hearing loss.