Introduction: Frailty and dementia are prevalent, interrelated conditions in aging populations, yet the temporal relationship between frailty progression and dementia risk remains underexplored. This study aimed to investigate how both baseline frailty and its subsequent changes over time are associated with the risk of incident dementia in 3 large prospective cohorts. Methods: This study analyzed data from 25,357 participants in 3 prospective cohorts: China Health and Retirement Longitudinal Study 2011–2020, U.S. Health and Retirement Study 2006–2020, and English Longitudinal Study of Ageing 2004–2023. Cox proportional hazard models were used to calculate the hazard ratio and 95% CI between frailty and risks of dementia after adjusting for potential confounders. Analyses were conducted in 2025. Results: Across all 3 cohorts, participants who were prefrail (hazard ratio=1.13–1.17) or frail (hazard ratio=1.37–1.54) at baseline had a significantly increased risk of dementia compared with their robust peers. This risk was most pronounced for those whose frailty status worsened over time; robust participants who transitioned to a frail state had increased risks of dementia (hazard ratio=2.09–2.77). Participants who transitioned from prefrail to frail showed significantly elevated risks of dementia (hazard ratio=1.32). Participants with the upper and middle tertiles of total frailty index and those in the upper tertile of change in frailty index exhibited significantly increased risks of dementia. Conclusions: The findings from 3 large, multinational cohorts consistently demonstrate a strong link between frailty progression and an increased risk of dementia particularly for those shifting from robust to frail state. This highlights that frailty is a globally relevant, modifiable target for dementia prevention, accentuating the necessity for proactive healthcare approaches to manage its onset and progression.