Frailty and social contact are significant factors influencing dementia risk. While previous studies have separately examined these factors, their combined impact on dementia remains underexplored.This study included 338,567 UK biobank participants from 2006 to 2010, with follow-up until December 2022. Additionally, 30,408 participants with brain magnetic resonance imaging data were analyzed for hippocampal volume. Cox proportional hazards regression and linear regression models were used to assess associations.The study followed 338,567 participants (mean [SD] age, 60.4 [5.2] years; 54.1 % men) for a median of 13.7 years, documenting 7362 cases of all-cause dementia. Both frailty and lower social contact independently increased the risk of all-cause dementia, Alzheimer's disease (AD), and vascular dementia (VaD). Compared to individuals with non-frailty and high social contact, those with lower social contact and higher frailty had a significantly increased risk of all-cause dementia, with the highest risk observed in individuals with frailty and low social contact (HR = 2.65, 95 % CI: 2.27-3.11). Similar patterns were found for AD and VaD. Furthermore, hippocampal volume was significantly reduced in individuals with frailty and low social contact (β = -0.24, 95 % CI: -0.43 to -0.06) compared to those with non-frailty and high social contact.The study predominantly included European descent individuals, with most frailty and social contact data based on baseline self-reports.The combination of frailty and low social contact is associated with the highest risk of dementia. These findings suggest that both physiological and social factors should be simultaneously considered in dementia prevention strategies.Copyright © 2025 Elsevier B.V. All rights reserved.