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Background: Observational studies have consistently found a link between low serum 25‐hydroxyvitamin D concentration and higher risk of cognitive impairment. Results from randomized controlled trials have been mixed, and few have been conducted in the general population. Methods: We recruited 21,315 community‐dwelling Australians aged between 60 and 84 years to participate in the D‐Health Trial, a randomized, double‐blind, placebo‐controlled trial. The intervention was monthly oral doses of 60,000 international units of vitamin D or placebo for 5 years. We assessed cognitive function in a randomly sampled group of participants aged ≥70 years using the Telephone Interview for Cognitive Status (TICS) at 2 and 5 years after randomization. The primary outcome for this analysis was TICS score; the secondary outcome was the proportion of people who had cognitive impairment (defined as TICS score ≤25). We analyzed data using mixed models (linear and logistic). Results: We interviewed 3887 participants at year 2 and 3614 participants at year 5. The mean TICS score at these time points was 32.3 and 32.2, respectively. Vitamin D supplementation did not affect cognitive function as measured by TICS score (mean difference between vitamin D and placebo groups 0.04; 95% CI −0.14 to 0.23), or alter risk of cognitive impairment (odds ratio 1.00; 95% CI 0.75 to 1.33). Conclusions: Monthly bolus doses of vitamin D supplementation neither enhanced nor hindered cognitive function among older adults. Population‐wide vitamin D supplementation of older adults that are largely vitamin D replete is unlikely to substantially benefit cognition. [ABSTRACT FROM AUTHOR]