Abstract Objective To investigate the association between long term residential exposure to road traffic and railway noise and risk of incident dementia. Design Nationwide prospective register based cohort study. Setting Denmark. Participants 1 938 994 adults aged ≥60 years living in Denmark between 1 January 2004 and 31 December 2017. Main outcome measures Incident cases of all cause dementia and dementia subtypes (Alzheimer’s disease, vascular dementia, and Parkinson’s disease related dementia), identified from national hospital and prescription registries. Results The study population included 103 500 participants with incident dementia, and of those, 31 219 received a diagnosis of Alzheimer’s disease, 8664 of vascular dementia, and 2192 of Parkinson’s disease related dementia. Using Cox regression models, 10 year mean exposure to road traffic and railway noise at the most (L _den max) and least (L _den min) exposed façades of buildings were associated with a higher risk of all cause dementia. These associations showed a general pattern of higher hazard ratios with higher noise exposure, but with a levelling off or even small declines in risk at higher noise levels. In subtype analyses, both road traffic noise and railway noise were associated with a higher risk of Alzheimer’s disease, with hazard ratios of 1.16 (95% confidence interval 1.11 to 1.22) for road L _den max ≥65 dB compared with <45 dB, 1.27 (1.22 to 1.34) for road L _den min ≥55 dB compared with <40 dB, 1.16 (1.10 to 1.23) for railway L _den max ≥60 dB compared with <40 dB, and 1.24 (1.17 to 1.30) for railway L _den min ≥50 dB compared with <40 dB. Road traffic, but not railway, noise was associated with an increased risk of vascular dementia. Results indicated associations between road traffic L _den min and Parkinson’s disease related dementia. Conclusions This nationwide cohort study found transportation noise to be associated with a higher risk of all cause dementia and dementia subtypes, especially Alzheimer’s disease.