This study aimed to explore the relationship between body mass index (BMI) and hearing loss. We analyzed data from the Korean National Health Insurance Service Health Screening Cohort 2009–2019 (291,471 patients with hearing loss and 6,088,979 control participants). Both patient groups were subsequently divided into four groups according to BMI: <18.5 (underweight), 18.5–24.9 (normal), 25–29.9 (obese I), and ≥30 (obese II). To evaluate the relationship between BMI and hearing loss, multivariate logistic regression analysis was used, adjusting for age, sex, smoking, alcohol consumption, blood pressure, triglycerides, total cholesterol, low-density lipoprotein, proteinuria, serum creatinine, aspartate aminotransferase, alanine aminotransferase, and fasting glucose levels. The adjusted odds ratio (OR) of the underweight group for hearing loss was 1.21 (95% CI = 1.19–1.24) compared to the normal BMI group, whereas the adjusted ORs of obese I and obese II groups for hearing loss were 0.95 and 0.87, respectively. Being underweight was generally associated with an increased prevalence of hearing loss in the Korean adult population.