Although some experimental biological studies have indicated that citrus may have preventive effects against cognitive impairment, no cohort study has yet examined the relationship between citrus consumption and incident dementia. In a baseline survey, we collected data on daily citrus intake (categorised as ≤2, 3-4 times/week or almost every day) and consumption of other foods using a FFQ, and used a self-reported questionnaire to collect data on other covariates. Data on incident dementia were retrieved from the Japanese Long-term Care Insurance database. A multivariate-adjusted Cox model was used to estimate the hazard ratios (HR) and 95 % CI for incident dementia according to citrus consumption. Among 13 373 participants, the 5·7-year incidence of dementia was 8·6 %. In comparison with participants who consumed citrus ≤2 times/week, the multivariate-adjusted HR for incident dementia among those did so 3-4 times/week and almost every day was 0·92 (95 % CI 0·80, 1·07) and 0·86 (95 % CI 0·73, 1·01), respectively (P trend=0·065). The inverse association persisted after excluding participants whose dementia events had occurred in the first 2 years of follow-up. The multivariate HR was 1·00 (reference) for ≤2 times/week, 0·82 (95 % CI 0·69, 0·98) for 3-4 times/week and 0·77 (95 % CI 0·64, 0·93) for almost every day (P trend=0·006). The present findings suggest that frequent citrus consumption was associated with a lower risk of incident dementia, even after adjustment for possible confounding factors.