Background: Dietary sodium may influence cognitive function through its effects on cerebrovascular function and cerebral blood flow. Methods: The aim of this study was to evaluate the association of dietary sodium intake with cognitive decline in community-dwelling older adults. We also evaluated the associations of dietary potassium and sodium:potassium intake with cognitive decline, and associations of these nutrients with micro- and macro-structural brain magnetic resonance imaging (MRI) indices. In all, 1,194 participants in the Health Aging and Body Composition study with measurements of dietary sodium intake (food frequency questionnaire [FFQ]) and change in the modified Mini Mental State Exam (3MS) were included. Results: The age of participants was 74 ± 3 years with a mean dietary sodium intake of 2,677 ± 1,060 mg/day. During follow-up (6.9 ± 0.1 years), 340 (28%) had a clinically significant decline in 3MS score (≥1.5 SD of mean decline). After adjustment, dietary sodium intake was not associated with odds of cognitive decline (OR 0.96, 95% CI 0.50–1.84 per doubling of sodium). Similarly, potassium was not associated with cognitive decline; however, higher sodium:potassium intake was associated with increased odds of cognitive decline (OR 2.02 [95% CI 1.01–4.03] per unit increase). Neither sodium or potassium alone nor sodium:potassium were associated with micro- or macro-structural brain MRI indices. These results are limited by the use of FFQ. Conclusions: In community-dwelling older adults, higher sodium:potassium, but not sodium or potassium intake alone, was associated with decline in cognitive function, with no associations observed with micro- and macro-structural brain MRI indices. These findings do not support reduction dietary sodium/increased potassium intake to prevent cognitive decline with aging.