INTRODUCTION: Potent lifestyle interventions to increase moderate-to-vigorous physical activity are urgently needed for population-level chronic disease prevention. This trial tested the independent and joint effects of a mobile health system automating adaptive goal setting and immediate financial reinforcement for increasing daily walking among insufficiently active adults. STUDY DESIGN: Participants were randomized into a 2 (adaptive versus static goal setting) x 2 (immediate versus delayed financial incentive timing) condition factorial trial to increase walking. SETTINGS/PARTICIPANTS: Participants (N = 512 adults) were recruited between 2016 and 2018 and were 64.5% female, aged 18 to 60 years, 18.8% Hispanic, 6.1% African American, and 83% White. INTERVENTION: Principles of reinforcement and behavioral economics directed intervention design. MAIN OUTCOME MEASURES: Participants wore accelerometers daily (133,876 day-level observations) that remotely measured moderate-to-vigorous physical activity bout minutes of >= 3 minutes/day for 1 year. Primary outcomes were between-condition differences in (1) engaging >= 1 bout of moderate-to-vigorous physical activity on each day and (2) on days with >= 1 bout, daily total moderate-to-vigorous physical activity minutes. RESULTS: Mixed-effects hurdle models tested treatment group x phase (time) interactions using an intent-to-treat approach in 2021. Engaging in any ambulatory moderate-to-vigorous physical activity was greater for adaptive than for static goal groups (OR 2.34, 95% CI 2.10 to 2.60 versus OR 1.66, 95% CI 1.50 to 1.84; p < 0.001) and for Immediate than for Static Reinforcement groups (OR 2.16 95% CI 1.94 to 2.40 versus OR 1.77, 95% CI 1.59 to 1.97; p < 0.01). The immediate reinforcement group increased by 16.54 moderate-to-vigorous physical activity minutes/day, whereas the Delayed Reinforcement group increased by 9.91 minutes/day (p < 0.001). The combined adaptive goals plus immediate reinforcement group increased by 16.52 moderate-to-vigorous physical activity minutes/day, significantly more than that of either Delayed Reinforcement group. CONCLUSIONS: This study offers automated and scalable-behavior change strategies for increasing walking among adults most at-risk for chronic diseases attributed to sedentary lifestyles. TRIAL REGISTRATION: www.ClinicalTrials.gov NCT02717663.

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