BACKGROUND: Sedentary behavior (SB) is associated with impaired physical function, falls, and higher mortality in older adults, which in turn leads to loss of physical independence. PURPOSE: The purpose of this feasibility study was 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 function residing in assisted living facilities. METHODS: SUN included two intervention groups: one group focused on reducing total sedentary time (SUN^SL); one group focused on increasing sit-to-stand (STS) transitions (SUN^STS). All participants (N=71; M_age=87±7yrs) received weekly health coaching 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 were used to examine the efficacy of SUN on outcome variables over time.RESULTS: Both 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. SUN^STS significantly increased STS transitions at 6 weeks (5.4±4.1, p<0.001) while SUN^SL had no changes (0.5±3.1, p>0.9). No 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 groups improved physical function 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 demonstrates the efficacy to improve SB and physical function and may be a promising strategy to maintain function for activities of daily living to prevent loss of independence in older adults.