To investigate the relationship between indoor radon level and stroke, which is a major factor for background radiation.This study combines 2 nationwide studies. Demographic characteristics and medical history of participants were obtained from Korean National Health and Nutrition Examination Survey (KNHANES) from 2007 to 2012. Participants over 40 years old and who completed the questionnaire were included in the study. Indoor radon concentration was analyzed using the mean value of winter housing radon concentration from 2012 to 2016 published by the National Institute of Environmental Research. The average values of each metropolitan city and province were assigned to the residence of the participant. To eliminate the potential confounding factors, participants' age, sex, hypertension, diabetes, dyslipidemia, ischemic heart disease, education level, occupation, smoking, drinking, exercise, and dietary intake were adjusted in multivariable logistic regression.Total of 28,557 participants were included in this study. Indoor radon levels were significantly higher in the participants with stroke, and the prevalence of stroke increased as indoor radon levels increased (P < .001, P for linear trend <.001). Indoor radon level was associated with stroke even after adjusting potential confounding factors (OR: 1.004 [95CI: 1.001-1.007], P = .010) and high radon exposure (indoor radon over 100Bq/m3) was also associated with stroke (OR: 1.242 [95CI: 1.069-1.444], P = .005). Trend analysis showed linear correlation of increased odds between radon quartile and stroke (P for linear trend < .001). In subgroup analysis, elevated indoor radon was most strongly associated in participants with age over 76(OR: 1.872[95%CI:1.320-2.654], P < .001).High indoor radon concentration may be associated with stroke. Specifically, elevated radon was associated with stroke in participants over 76 years old. In high-risk population, home modification to reduce indoor radon may help decreasing the risk of stroke.