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Background: The expected increase of dementia prevalence in the coming decades will mainly be in low-income and middle-income countries and in people with low socioeconomic status in high-income countries. This study aims to reduce dementia risk factors in underserved populations at high-risk using a coach-supported mobile health (mHealth) intervention., Methods: This open-label, blinded endpoint, hybrid effectiveness-implementation randomised controlled trial (RCT) investigated whether a coach-supported mHealth intervention can reduce dementia risk in people aged 55-75 years of low socioeconomic status in the UK or from the general population in China with at least two dementia risk factors. The primary effectiveness outcome was change in cardiovascular risk factors, ageing, and incidence of dementia (CAIDE) risk score from baseline to after 12-18 months of intervention. Implementation outcomes were coverage, adoption, sustainability, appropriateness, acceptability, fidelity, feasibility, and costs assessed using a mixed-methods approach. All participants with complete data on the primary outcome, without imputation of missing outcomes were included in the analysis (intention-to-treat principle). This trial is registered with ISRCTN, ISRCTN15986016, and is completed., Findings: Between Jan 15, 2021, and April 18, 2023, 1488 people (601 male and 887 female) were randomly assigned (734 to intervention and 754 to control), with 1229 (83%) of 1488 available for analysis of the primary effectiveness outcome. After a mean follow-up of 16 months (SD 2·5), the mean CAIDE score improved 0·16 points in the intervention group versus 0·01 in the control group (mean difference -0·16, 95% CI -0·29 to -0·03). 1533 (10%) invited individuals responded; of the intervention participants, 593 (81%) of 734 adopted the intervention and 367 (50%) of 734 continued active participation throughout the study. Perceived appropriateness (85%), acceptability (81%), and fidelity (79%) were good, with fair overall feasibility (53% of intervention participants and 58% of coaches), at low cost. No differences in adverse events between study arms were found., Interpretation: A coach-supported mHealth intervention is modestly effective in reducing dementia risk factors in those with low socioeconomic status in the UK and any socioeconomic status in China. Implementation is challenging in these populations, but those reached actively participated. Whether this intervention will result in less cognitive decline and dementia requires a larger RCT with long follow-up., Funding: EU Horizon 2020 Research and Innovation Programme and the National Key R&D Programmes of China., Translation: For the Mandarin translation of the abstract see Supplementary Materials section., Competing Interests: Declaration of interests AW received research grants from EU IMI2 (MOPEAD), JPND (ADDITION, EURO-FINGER, PMI-AD), IHI (PROMINENT), and the Swedish VINNOVA program (PREDEM)(all paid to institution) and is licence holder of RUD-instrument (in part); outside the submitted work. RH received research grants from JPND, ZonMW, IMI, H2020 (paid to institution); received consulting fees from Lilly Nederland, iMTA, Biogen Nederlands, Biogen MA, Eisai (paid to institution); is a member of ISPOR special interest group open-source models, IPECAD modelling group and Alzheimer Europe Expert Advisory Panel (unpaid); outside the submitted work. SA received grants from EU (Institutional grant [Horizon 2020 Research and Innovation Programme agreement 779238]); Region Occitanie/Pyrénées-Méditerranée (1901175); the European Regional Development Fund (MP0022856); MSD Avenir Inspire Chairs of Excellence (Alzheimer Prevention in Occitania and Catalonia, EDENIS, KORIAN, Pfizer, and Pierre-Fabre); AXA Personal (current); Biogen Personal (2022); Roche Personal (2021); Leventis foundation (2022); ADI (2022); and has a leadership role in the French Alzheimer association (Scientific Committee); outside the submitted work. All other authors declare no competing interests., (Copyright © 2024 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY-NC-ND 4.0 license. Published by Elsevier Ltd.. All rights reserved.)