Background: Overweight and obese older adults have high sedentary time. We tested the feasibility and preliminary effects of a sedentary time reduction intervention among adults over age 60 with a body mass index over 27 kg/m2 using a nonrandomized one-arm design. Methods: Participants (N = 25, mean age = 71.4, mean body mass index = 34) completed an 8-week theory-based intervention targeting reduced total sitting time and increased sit-to-stand transitions. An inclinometer (activPAL™) measured the primary outcomes, change in total sitting time and sit-to-stand transitions. Secondary outcomes included physical activity (ActiGraph GT3X+ accelerometer), self-reported sedentary behaviors, physical function (Short Physical Performance Battery), depressive symptoms (8-item Patient Health Questionnaire), quality of life (PROMIS), and study satisfaction. Paired t tests examined pre-post test changes in sitting time, sit-to-stand transitions, and secondary outcomes. Results: Inclinometer measured sitting time decreased by 27 min/day (p < 0.05) and sit-to-stand transitions increased by 2 per day (p > 0.05), while standing time increased by 25 min/day (p < 0.05). Accelerometer measured sedentary time, light-intensity, and moderate-to-vigorous physical activity improved (all p values = 0.05). Self-reported sitting time, gait speed, and depressive symptoms also improved (all p values < 0.05). Effect sizes were small. Study satisfaction was high. Conclusions: Reducing sitting time is feasible, and the intervention shows preliminary evidence of effectiveness among older adults with overweight and obesity. Randomized trials of sedentary behavior reduction in overweight and obese older adults, most of whom have multiple chronic conditions, may be promising.