Cancer patients may be at decreased risk of Alzheimer's disease. This hypothesis is best developed for non-melanoma skin cancer (NMSC), but supportive epidemiological data are sparse. We therefore conducted a nationwide cohort study of the association between NMSC and Alzheimer's disease (main outcome) and all-cause dementia. Using Danish medical databases, we identified adults diagnosed with NMSC between 1 January 1980 and 30 November 2013 (n = 216,221) and a comparison cohort of five individuals matched to each NMSC patient by sex and birth year (n = 1,081,097). We followed individuals from the time of diagnosis, or corresponding date for matched comparators, until a dementia diagnosis, death, emigration, or 30 November 2013, whichever came first. We used stratified Cox regression adjusted for comorbidities to compute hazard ratios (HRs) associating NMSC with dementia. We computed cumulative risks of dementia, treating death as a competing risk. NMSC was associated with a HR of 0.95 (95% confidence interval [CI]: 0.92-0.98) for Alzheimer's disease and 0.92 (95% CI: 0.90-0.94) for all-cause dementia. HRs were similar for basal cell and squamous cell carcinoma, the two most common forms of NMSC. Estimates of risk reduction were more pronounced in the beginning of follow-up, reaching null after 5-10 years. At the end of follow-up (34 years), cumulative risk of Alzheimer's disease was 4.6% (95% CI: 4.4%-4.8%) among patients with NMSC vs. 4.7% (95% CI: 4.6%-4.9%) in the comparison cohort. In conclusion, NMSC was associated with 2%-10% reductions in relative risks of Alzheimer's disease and all-cause dementia. However, these small inverse associations may have been caused by ascertainment bias due to decreased awareness of NMSC tumors in persons with undiagnosed early cognitive impairment or by confounding from a more neuroprotective lifestyle among persons with NMSC.