Aim Evidence for the association between dialysis treatment and cognitive decline is limited. The present study aimed to determine whether dialysis treatment is associated with cognitive decline in adult outpatients of a general hospital in Japan. Methods This was a cross‐sectional substudy of the Project in Sado for Total Health (PROST). Total Health PROST targeted adult outpatients of a general hospital in Sado City, Niigata, Japan. Among 753 patients (mean age 68.1 ± 11.6 years) analyzed, 66 received dialysis. Cognitive state was evaluated using the Mini‐Mental State Examination, and those with a Mini‐Mental State Examination score <24 were considered “cognitively declined.” The prevalence of cognitive decline was compared by odds ratios calculated with multiple logistic regression analysis. Variables included in the analyses were dialysis, age, sex and self‐reported histories of hypertension, diabetes, stroke and ischemic heart disease. Results Of the 66 dialysis patients, 24 (36.4%) showed cognitive decline, whereas 172 (25.0%) of 687 non‐dialysis patients showed cognitive decline. The age and sex‐adjusted odds ratio for cognitive decline in dialysis patients was 2.57 (95% confidence interval 1.43–4.61), relative to non‐dialysis patients. The odds ratio remained significant (odds ratio 2.69, 95% confidence interval 1.49–4.88) even after adjusting for all covariates. Conclusion The prevalence of cognitive decline was high in dialysis patients relative to non‐dialysis patients among outpatients of a general hospital in Japan. Geriatr Gerontol Int 2016; ••: ••–••.