The impact of underweight on the risk of developing primary open-angle glaucoma (POAG) is not known, although the association between obesity and POAG has been well studied. We evaluated the risk of POAG among underweight patients by studying a nationwide cohort sample in South Korea.We analyzed data from the Korean National Health Insurance Research Database collected between 2009 and 2012 for 17,000,636 patients aged 40 years or older. Newly diagnosed POAG in the cohort was identified using claims data between 2009 and 2015.A total of 442,829 individuals (2.60%) were classified as underweight (body mass index [BMI] < 18.5 kg/m). During the follow-up period, 435,756 (2.56%) subjects were newly diagnosed with POAG. Multivariate analyses revealed that underweight was significantly related to an increased risk of future POAG development, by 9.8% and 27.8% in individuals with and without diabetes, respectively. There was a reverse J-shaped relationship between BMI and risk of POAG in the normal, impaired glucose tolerance, and diabetes groups; especially, this relationship was most notable in participants with diabetes.Patients who were underweight exhibited a significantly higher prospective risk of POAG, even after adjusting for confounding factors.