<p>The biological aging acceleration predicts both morbidity and mortality, while few investigations have examined its utility in evaluating transitions, e.g. development patterns, of atrial fibrillation (AF) and dementia. We aimed to investigate the utility of biological aging acceleration in predicting transitions of AF and dementia. A total of 402,955 participants (mean [SD] age: 56.5 [8.1] years; men: 45.9%) in the UK Biobank were included. Biological age was calculated using the phenotypic age, based on the chronological age and 9 biomarkers. A multi-state survival analysis was conducted to examine transition patterns between AF and dementia. The increased biological aging acceleration was consistently associated with all transition patterns between AF and dementia, including transitions from incident AF or dementia to the comorbidity. Strong linear associations were observed. Our findings highlight the overlooked utility of biological aging acceleration in evaluating the transitions of AF and dementia in the general population.</p>