OBJECTIVE Facial nerve decompression is a salvage treatment for Bell's palsy patients for whom a poor prognosis is anticipated with standard medical treatment. The transmastoid approach is a frequently performed approach, but it remains unknown if this surgery is effective when the ossicular chain is preserved. This study aimed to determine the efficacy of facial nerve decompression using the transmastoid approach in Bell's palsy. METHODS This retrospective study included patients who had undergone transmastoid facial nerve decompression with ossicular chain preservation and patients who met the criteria for surgery, but received only medical treatment between January 2007 and May 2019, at a single centre. METHODS Attainment of House-Brackmann grade I at 12 months after onset of facial palsy. RESULTS The recovery rate to House-Brackmann grade I in the decompression group in the early phase (≤18 days after onset) was higher than that of the medical treatment group, although the difference was not significant (70% vs 47%, P = .160). However, within this early surgery group, a subgroup of cases with ≥95% facial nerve degeneration demonstrated a significant improvement in recovery rate (73% vs 30%, P = .018). Among surgeries performed in the late phase (≥19 days), only a subgroup with ≥95% facial nerve degeneration was available for analysis, and the difference in recovery rate was not significant compared with medical treatment alone (26% vs 30%, P = 1.00). Post-surgical hearing evaluation demonstrated that average hearing deterioration was 1.3 dB which was non-significant, suggesting this procedure does not cause hearing loss. CONCLUSIONS Transmastoid facial nerve decompression with ossicular chain preservation in the early phase after symptom-onset is an effective salvage treatment for severe Bell's palsy with ≥95% facial nerve degeneration.