Purpose: While the benefits of adopting a more plant-based diet for sustainability and animal welfare are clear, its long-term health impacts, including the impact on cognitive ageing, are limited studied. Therefore, we investigated the associations between plant-based diet adherence and cognitive ageing. Methods: Data from a previous intervention study involving community-dwelling adults aged ≥ 65 years were analysed at baseline (n = 658) and after 2-year follow-up (n = 314). Global and domain-specific cognitive functioning were assessed at both timepoints. Overall, healthful and unhealthful plant-based dietary indices were calculated from a 190-item food frequency questionnaire. Multivariate-adjusted linear regression models were applied to test for associations. Results: After full-adjustment, higher overall adherence to a plant-based diet was not associated with global cognitive function (difference in Z-score, tertile 1 versus 3 [95% CI]: 0.04 [− 0.05, 0.13] p = 0.40) or cognitive change (− 0.04 [− 0.11, 0.04], p = 0.35). Similarly, healthful and unhealthful plant-based diet indices were not associated with cognitive functioning (respectively p = 0.48; p = 0.87) or change (respectively p = 0.21, p = 0.33). Interestingly, we observed fish consumption to influence the association between plant-based diet adherence and cognitive functioning (p-interaction = 0.01), with only individuals with a fish consumption of ≥ 0.93 portion/week benefitting from better overall plant-based diet adherence (β per 10-point increment [95% CI]: 0.12 [0.03, 0.21] p = 0.01). Conclusion: We did not demonstrate associations of a more plant-based diet with cognitive ageing. However, possibly such association exists in a subpopulation with higher fish intake. This would be in line with earlier observations that diets rich in plant foods and fish, such as the Mediterranean diet, may be beneficial for cognitive ageing. Trial registration: Registered at clinicaltrials.gov (NCT00696514) on June 12, 2008.