Abstract Background Studies of the relationship between vitamin D and physical functioning have had inconsistent results. Methods Physical functioning measures were collected for up to 2 years during a 2-stage, Bayesian, response-adaptive, randomized trial of 4 doses of vitamin D3 supplementation (200 [control], 1 000, 2 000, and 4 000 IU/day) to prevent falls. Two community-based research units enrolled adults aged ≥70 years, with elevated fall risk and serum 25-hydroxyvitamin D level of 10–29 ng/mL. The Pooled Higher Doses (PHD) group (≥1 000 IU/day, n = 349) was compared to the control group (n = 339) on changes in Short Physical Performance Battery (SPPB) score and its component tests, Timed Up-and-Go (TUG) test, 6-minute walk distance, and grip strength. Results The trial enrolled 688 participants. Mean age was 77.2 years, 56.4% were male, 79.7% White, and 18.2% Black. While the PHD and control groups both lost function over time on most outcomes, the 2 groups did not show differential change overall on any outcome. Incidence of transitioning to poor functioning on gait speed, SPPB score, or TUG test did not differ by dose group. Conclusion In older persons with low serum 25-hydroxyvitamin D level and elevated fall risk, high-dose vitamin D supplementation, ≥1 000 IU/day, did not improve measures of physical function compared to 200 IU/day. Clinical Trial Registration NCT02166333