Post-traumatic stress disorder (PTSD) has been identified as a potential risk factor for developing dementia. There are currently, however, no meta-analyses quantifying this risk.To systematically review and quantify the risk of future dementia associated with PTSD across populations. PROSPERO registration number CRD42019130392.We searched nine electronic databases up to 25 October 2019 for longitudinal studies assessing PTSD and risk of dementia. We used random- and fixed-effects meta-analyses to pool estimates across studies.PTSD was associated with a significant risk for all-cause dementia: pooled hazard ratio HR = 1.61 (95% CI 1.43-1.81, I2= 85.8%, P < 0.001; n = 1 693 678; 8 studies). Pooled HR was 1.61 (95% CI 1.46-1.78; I2= 80.9%, P < 0.001; n = 905 896; 5 studies) in veterans, and 2.11 (95% CI 1.03-4.33, I2= 91.2%, P < 0.001; n = 787 782; 3 studies) in the general population. The association between PTSD and dementia remained significant after excluding studies with high risk of bias (HR = 1.55, 95% CI 1.39-1.73, I2= 83.9%, P < 0.001; n = 1 684 928; 7 studies). Most studies included were retrospective and there was evidence of high heterogeneity.This is the first meta-analysis quantifying the association of PTSD and risk of dementia showing that PTSD is a strong and potentially modifiable risk factor for all-cause dementia. Future studies investigating potential causal mechanisms, and the protective value of treating PTSD are needed.