SUMMARY Latinx older adults are at higher risk for developing Alzheimer’s Disease and Related Dementias (ADRD) than White older adults. Despite older Latinx adults having lower rates of physical activity (PA) and a high burden of comorbidities that are associated with ADRD, few studies have targeted this population with theory-based and culturally adapted approaches to reduce risk for ADRD. Building on pilot work by the team, the proposed project would allow us to culturally adapt a theory-based intervention to promote moderate-to-vigorous physical activity (MVPA) for Latinx adults ages 55 to 89 years. In Stage I (R61/Aim 1), we will finalize the recruitment and intervention strategy (including finalizing the target mechanisms of adherence to MVPA, initial cultural adaptation and translation) through meetings with our community advisory board and pilot testing with 10 Latinx adults. In Stage II (R33/Aim 2), we will randomize 130 Latinx adults to receive the 16-week, fully remote De Pié y a Movernos (De Pié) intervention or a control condition. The intervention will target the mechanisms of self-efficacy, PA habit strength, social support, and PA enjoyment using a staircase approach where participants will initially reduce their sitting by moving more to build confidence towards engaging in MVPA. Participants will also receive a Fitbit activity tracker, health coaching contacts, feedback on meeting goals, and will identify a support person. We will determine the effect of the De Pié intervention on activPAL accelerometer-based MVPA minutes/day (primary outcome) and adherence to meeting the MVPA guidelines of 150 minutes per week (secondary outcome). Further, mediation analyses will be conducted to determine the effect of the intervention on mechanisms of adherence (self-efficacy, habit strength, social support, enjoyment) and identify whether these mechanisms explain intervention effects on MVPA minutes/week and adherence to MVPA guidelines. We will explore cognition, acculturation, age and sex as moderators of these associations. Results will help identify important mechanisms of adherence to MVPA and inform culturally adapted, theory- based approaches to prevent ADRD in the Latinx community.