INTRODUCTION: Physical activity is associated with reduced risk of cognitive and functional decline but scalable, sustainable interventions for populations at risk for Alzheimers disease (AD) and AD and related dementias (ADRD) are lacking. METHODS: A 12-week randomized-controlled trial was conducted with a 3-week follow-up using a national AD prevention registry (GeneMatch). The control group (n = 50) set step goals and received daily feedback. The intervention group (n = 44) also received a behaviorally designed game based on achieving step goals and reinforced by a support partner. RESULTS: Intervention participants (94 participants, mean age 70, 78% female) had greater change in mean daily step count than control of 1699 steps/day (95% confidence interval [CI], 1149-2249), P < 0.0001, which was sustained in the follow-up period at 1219 steps/day (95% CI, 455-1983), P = 0.0018. Carriers of the apolipoprotein E ε4 gene (high risk) did not perform differently than non-carriers; however, high self-reported risk perception was associated with higher activity. DISCUSSION: A gamified intervention was effective in promoting and sustaining higher physical activity in older adults at genetic risk for AD/ADRD. HIGHLIGHTS: A simple game played with a support partner increased walking in older adults at risk for Alzheimers disease (AD). The game also increased minutes of moderate-to-vigorous physical activity per day. Perception of lifelong AD risk was associated with increased activity but genetic risk (apolipoprotein E ε4+) was not. TRIAL REGISTRATION: ClinicalTrials.gov: NCT05069155.