Objective: We investigated the risk of sudden sensorineural hearing loss (SSNHL) in patients with autoimmune diseases compared with a control group in a population-based study using a National Health Insurance Service National Sample Cohort data from Korea. Methods: We enrolled autoimmune-disease patients who were ≥20 years of age in 2006 into the autoimmune-disease group, and selected a control group with similar demographic characteristics. We tracked the 2 groups from 2006 to 2015 and compared the proportions of patients who developed SSNHL. Results: Of 13,250 patients in the autoimmune-disease group, 145 experienced an SSNHL event (1.09%). Of the 66,250 in the control group, 484 experienced an SSNHL event (0.73%). The SSNHL risk was significantly higher in the autoimmune-disease group than in the control group. SSNHL incidence was significantly higher among patients with antiphospholipid syndrome (APS), multiple sclerosis (MS), rheumatoid arthritis (RA), and connective-tissue diseases including Sjögren syndrome and Behçet disease. In detailed stratified analyses using reinforced additional diagnostic codes, only RA patients had a significantly higher SSNHL incidence than the control group. Conclusion: The association of several autoimmune diseases with SSNHL was evaluated in this large-scale, population-based, big-data study. The risk of SSNHL was significantly higher in patients with APS, MS, RA, and connective-tissue diseases including Sjögren syndrome and Behçet disease than in patients without autoimmune diseases. SSNHL, in particular, was significantly associated with RA according to detailed analyses using reinforced additional diagnostic codes.