Objective To examine the efficacy of body brain life (BBL), a 12-week online dementia risk reduction intervention. Methods BBL was evaluated in a randomized controlled trial in 176 middle-aged adults with >2 risk factors and <2 protective factors for Alzheimer's disease (AD) assessed on a brief screening instrument. Participants were randomized to BBL, BBL plus face-to-face group sessions (BBL + FF) or active control (control). Score on the Australian National University-Alzheimer's disease risk index (ANU-ADRI), a validated index of AD risk, was the primary outcome measure assessed at baseline, 12, and 26 weeks. Results A group by time interaction at 26 weeks showed a significant reduction in ANU-ADRI score for BBL compared with control. Planned contrasts showed the BBL and BBL + FF groups had improvement in ANU-ADRI scores at 12 weeks (BBL + FF: z = -0.25; P =.021; BBL: z = -0.25; P =.008) and 26 weeks (BBL + FF: z = -0.48; P <.001; BBL: z = -0.28; P =.004) due to increase in protective factors. Conclusions This short intervention resulted in dementia risk reduction. Online dementia risk reduction interventions show promise for reducing the overall dementia risk in middle-aged adults with multiple risk factors. Clinical Trial Registration: The study is registered under Trial Registration: Reg. # ACTRN12612000147886.