<p>Dementia is a growing global public health concern, with an increasing number of individuals affected due to the aging population. Although chronic inflammation is implicated in cognitive impairment, studies on its association with autoimmune diseases, which are representative chronic inflammatory diseases, are lacking. This study aimed to investigate whether the onset of autoimmune diseases is associated with an increased risk of dementia or cognitive impairment. This study used data from the Korean National Health Insurance Service–Health Screening from 2002 to 2019, with 8,743,801 person-years. The risk of dementia according to the type of autoimmune disease was calculated using the Cox proportional hazards model. The status and risk of cognitive impairment, which were assessed using the Korean Dementia Screening Questionnaire-Prescreening related with autoimmune diseases also estimated. Among participants, 8.3% developed dementia. All autoimmune diseases significantly increased dementia risk (hazard ratio [HR] 1.32; 95% confidence interval [CI] 1.29–1.35), particularly Alzheimer’s disease (HR 1.36; 95% CI, 1.32–1.40), vascular dementia (HR, 1.21; 95% CI, 1.12–1.30), and unspecified dementia (HR, 1.25; 95% CI, 1.18–1.33). Autoimmune diseases also increased the risk of positive dementia screening (odds ratio [OR], 1.18; 95% CI, 1.12–1.24), particularly connective tissue disorders (OR, 1.24; 95% CI, 1.15–1.34). This large-scale cohort study demonstrates a significant association between autoimmune diseases and increased risk of dementia and cognitive impairment. These findings underscore the potential importance of chronic inflammation in dementia pathogenesis and highlight the need for early cognitive screening and intervention strategies in patients with autoimmune diseases.</p>