Background: Sedentary behavior (SB) is associated with impaired physical function, frailty, falls, and higher mortality in older adults, the most sedentary age demographic in the US. There is limited research on the efficacy of interventions to reduce SB and improve physical function and mobility in older adults of moderate-to-low physical function. Purpose: To examine the efficacy of a 12-week intervention, Stand Up Now (SUN), to reduce SB and improve physical function and mobility in older adults of moderate-to-low physical function in Continuing Care Retirement Communities (CCRCs). Methods: SUN was conducted in four CCRCs that were randomized to two intervention groups with different SB change goals: one group focused on reducing total sedentary time (SUNSL) and one group focused on increasing sit-to-stand (STS) transitions (SUNSTS). Participants (N = 71; Mage=87±7yrs) in both SUN groups had weekly health coaching sessions over 12 weeks. SB, physical function, and mobility were measured at baseline, 6, and 12 weeks via the activPAL, Short Physical Performance Battery (SPPB), and the 8-foot up-and-go (8ft UG), respectively. Linear Mixed Models (LMM) were used to examine the efficacy of SUN on activPAL measured variables (i.e., sitting, standing, STS transitions, stepping, and steps), physical function, and mobility at 6 and 12 weeks. Results: Both SUN groups significantly decreased sedentary time (1.3±0.3hrs, p<0.001) and increased standing time (0.5±0.2hrs, p<0.02) at 6 weeks that was maintained at 12 weeks, compared to their baseline. SUNSTS significantly increased STS transitions at 6 weeks (5.4±4.1, p<0.001) while SUNSL had no changes in STS transitions (0.5±3.1, p>0.9). No significant changes were noted in stepping time (0.04±0.08hrs, p<0.15) or steps (261±234, p<0.14) per day in either group. Both SUN groups improved physical function (SPPB) from baseline to 6 weeks (1.5±0.4 points, p<0.001) that was maintained at 12 weeks. No significant changes were seen in mobility for either group (0.5±1.5sec, p>0.05). Conclusions: SUN demonstrated the efficacy to improve SB and physical function in older adults of moderate-to-low physical function. Interventions targeting SB, such as SUN, are a promising strategy to improve physical function needed for activities of daily living and extend independence in older adults.