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BACKGROUND Little is known about the long-term cognitive impact of internet usage among older adults. This research characterized the association between various measures of internet usage and dementia. METHODS We followed dementia-free adults aged 50 to 64.9 for a maximum of 17.1 (median=7.9) years using the Health and Retirement Study. The association between time-to-dementia and baseline internet usage was examined using cause-specific Cox models, adjusting for delayed entry and covariates. We also examined the interaction between internet usage and education, race-ethnicity, sex, and generation. Furthermore, we examined whether the risk of dementia varies by the cumulative period of regular internet usage to see if starting or continuing usage in old age modulates subsequent risk. Finally, we examined the association between the risk of dementia and daily hours of usage. Analyses were conducted from September 2021 to November 2022. RESULTS In 18,154 adults, regular internet usage was associated with approximately half the risk of dementia compared to non-regular usage, CHR (Cause-specific Hazard Ratio)=0.57, 95%CI=0.46-0.71. The association was maintained after adjustments for self-selection into baseline usage (CHR=0.54, 95%CI=0.41-0.72) and signs of cognitive decline at the baseline (CHR=0.62, 95%CI=0.46-0.85). The difference in risk between regular and non-regular users did not vary by educational attainment, race-ethnicity, sex, and generation. In addition, additional periods of regular usage were associated with significantly reduced dementia risk, CHR=0.80, 95%CI=0.68-0.95. However, estimates for daily hours of usage suggested a U-shaped relationship with dementia incidence. The lowest risk was observed among adults with 0.1-2 hours of usage, though estimates were non-significant due to small sample sizes. CONCLUSIONS Regular internet users experienced approximately half the risk of dementia than non-regular users. Being a regular internet user for longer periods in late adulthood was associated with delayed cognitive impairment, although further evidence is needed on potential adverse effects of excessive usage. This article is protected by copyright. All rights reserved.