Abstract Background In cardiomyopathies, obesity is known to affect the cardiac remodeling and is associated with a poor prognosis. However, in hypertrophic cardiomyopathy, which is included in the category of heart failure with preserved ejection fraction, the risk of underweight has been suggested, and its impact in practice is controversial. Purpose The aim of this study was to investigate the potential relationship between obesity and cardiovascular outcomes in patients diagnosed with hypertrophic cardiomyopathy. Methods We conducted a retrospective national cohort study from 2013 to 2020 utilizing administrative claims data and health check-up database from the Korean National Health Insurance Service. Patients newly diagnosed with hypertrophic cardiomyopathy from January 2015 to December 2019 were included. Body mass index was stratified into five groups based on the World Health Organization's recommended criteria for Asian populations, and patients were followed until the date of each outcome or the end of the study period. Predefined clinical outcomes included heart failure hospitalization events, fatal arrest events, all-cause death, and composite clinical outcomes. Results A total of 6,361 patients were included in the study, with a mean age was 60 ± 13 years and 32.6% female. During a median follow-up of 3.0 years (interquartile range, 1.8 to 4.3), 921 heart failure hospitalization, 185 fatal arrest, and 340 deaths occurred. The underweight group had higher adjusted hazard ratios for heart failure hospitalization, fatal arrest, all-cause death, and the composite outcome compared to the normal range body mass index group (HR 1.69, 95% CI 1.16-2.47; HR 2.29, 95% CI 1.06-4.93; HR 2.74, 95% CI 1.77-4.24; HR 1.77, 95% CI 1.28-2.45). In addition, the adjusted hazard ratios for the composite outcome were lower in the overweight and obese I group compared to the normal range (HR 0.80, 95% CI 0.68-0.95; HR 0.84, 95% CI 0.72-0.98). Conclusions Among patients with hypertrophic cardiomyopathy, those classified as underweight had an increased risk of heart failure hospitalization, fatal arrest, all-cause death, and composite outcomes compared with normal weight patients, while those classified as overweight and obese had a decreased risk of composite outcomes.