Despite the rapid rise in IBD, population-level familial risk estimates of inflammatory bowel diseases (IBD) are still lacking in Asian-Pacific countries. We aimed to quantify the familial risk of incident IBD among first-degree relatives (FDRs) of individuals with IBD according to age, sex and familial relationship.Using the South Korea National Health Insurance database (2002-2017), which has complete population coverage and confirmed accuracy of both FDR information and IBD diagnoses, we constructed a cohort of 21,940,795 study subjects comprising 12 million distinct families. We calculated incidence risk ratios of ulcerative colitis(UC) or Crohn's disease(CD) in individuals of affected FDRs compared to individuals without affected FDRs.Of 45,717 individuals with UC and 17,848 individuals with CD, 3.8% and 3.1% represented familial cases, respectively. Overall, there was a 10.2-fold (95% CI:9.39-11.1) and 22.1-fold (95% CI:20.5-24.5) significantly higher adjusted risk of UC and CD among FDRs of individuals with vs without IBD. Familial risk was highest among twins, followed by non-twin siblings, and then offspring of affected parents. Familial risk was generally higher within generation (sibling-sibling) vs between generations (parent-offspring). Familial risk also increased with increasing number of affected FDRs.According to this population-based analysis, there is a substantially increased risk of IBD among FDRs of affected individuals, with the highest risk among siblings and for CD. These findings might help with earlier diagnosis and appropriate therapeutic intervention in FDRs of individuals with IBD. Dedicated studies are needed, to evaluate the contributions of shared early-in life environmental exposures and genetic factors.