This study evaluates the prognostic value of tau positron emission tomography (tau PET) in predicting clinical progression from mild cognitive impairment (MCI) to dementia. The research is a multicenter cohort study with external validation, involving participants from South Korea, Sweden, the US, and Switzerland. The study followed 448 individuals with MCI for an average of 2.0 years. The primary outcomes were clinical progression to all-cause dementia or Alzheimer's disease (AD) dementia. Tau PET, amyloid-β (Aβ) PET, and magnetic resonance imaging (MRI) were assessed using quantitative thresholds and visual reads. The results showed that only tau PET predicted all-cause dementia better than a base model including demographic characteristics (AUC, 0.75; 95% CI, 0.70-0.80). In the validation cohort, tau PET replicated its predictive ability for all-cause dementia. For AD dementia, tau PET and visual reads also showed improved prediction compared to the base model. The study concludes that tau PET is the best stand-alone marker for predicting progression to dementia among individuals with MCI, suggesting it may be the most effective neuroimaging marker for prognostic purposes in MCI.This study evaluates the prognostic value of tau positron emission tomography (tau PET) in predicting clinical progression from mild cognitive impairment (MCI) to dementia. The research is a multicenter cohort study with external validation, involving participants from South Korea, Sweden, the US, and Switzerland. The study followed 448 individuals with MCI for an average of 2.0 years. The primary outcomes were clinical progression to all-cause dementia or Alzheimer's disease (AD) dementia. Tau PET, amyloid-β (Aβ) PET, and magnetic resonance imaging (MRI) were assessed using quantitative thresholds and visual reads. The results showed that only tau PET predicted all-cause dementia better than a base model including demographic characteristics (AUC, 0.75; 95% CI, 0.70-0.80). In the validation cohort, tau PET replicated its predictive ability for all-cause dementia. For AD dementia, tau PET and visual reads also showed improved prediction compared to the base model. The study concludes that tau PET is the best stand-alone marker for predicting progression to dementia among individuals with MCI, suggesting it may be the most effective neuroimaging marker for prognostic purposes in MCI.