The aim of this study is to investigate the incidence and mortality of Terson syndrome in patients with treated subarachnoid hemorrhage (SAH) in South Korea. In this nationwide, population-based study, we used the National Health Insurance(NHI) database (2011-2015) to identify patients aged ≥18 years. Newly diagnosed non-traumatic SAH, treated using clipping or coil embolization, were identified, and Terson syndrome was defined as newly diagnosed retinal or vitreous hemorrhage within 3 months of SAH diagnosis. We identified 22,864 patients with treated SAH (tSAH), 196 of whom had Terson syndrome, with the cumulative incidence during 5 years of 0.86% (95% CI: 0.74-0.98): 1.10% (95% CI: 0.88-1.33) in men and 0.71% (95% CI, 0.58-0.85) in women. The cumulative incidence of Terson syndrome in patients aged under 40 was higher than in those aged 40 or over (1.41% vs. 0.81%; p = 0.007). The mortality rate of Terson syndrome in patients with tSAH was not different from that in those without Terson syndrome (4.08% vs. 7.30%; p = 0.089). This was the first nationwide epidemiological study of Terson syndrome using a population-based database. The incidence of Terson syndrome in patients with tSAH was higher in those age under 40 than in those aged 40 or over.