Background and Objectives Now more than two-thirds of cancer survivors are aged 65 years or older, but evidence about their long-term health is thin. Cancer and its treatments have been linked to accelerated aging, so there is a concern that aging cancer survivors have an increased risk of age-related diseases, including dementia. Methods We examined the risk of dementia among 5-year breast cancer survivors using a matched cohort study design. We included breast cancer survivors aged 50 years and older at diagnosis (n = 26,741) and cancer-free comparison participants (n = 249,540). Women eligible for inclusion in the study were those born 1935–1975 and registered in the Swedish Total Population Register between January 1, 1991, and December 31, 2015. We defined breast cancer survivors as women with an initial breast cancer diagnosis between 1991 and 2005 who survived 5 or more years after their first diagnosis. We assessed all-cause dementia, Alzheimer disease (AD), and vascular dement