Objectives <p>This study aimed to evaluate longitudinal changes in relative sit-to-stand power (STS_power) in older adults and investigate contributing factors to its decline.</p> Methods <p>565 older adults (143 men and 422 women; ≥ 65 years) were tested at baseline and after 8 years of follow-up. Relative STS_power was estimated from the 30-s STS test using the Alcazar equation while body composition was assessed using bioelectrical impedance analysis. The basic components of relative STS_power (specific STS_power, leg muscle mass, body fat mass and fat-free mass) were calculated. Factors such as medication intake and daily walking and sitting time were recorded.</p> Results <p>Relative STS_power declined significantly in both sexes, with greater reductions in men than women (11.8 vs 8.0%, <i>p</i> = 0.024). Losses were greater in men and women aged ≥75 years (− 2.0% and − 2.0%∙year⁻¹, respectively) compared to those aged 65–69 years (− 1.1% and − 0.7% year⁻¹, respectively). Approximately 95% of the average decline in relative STS_power was attributed to reductions in specific STS_power, while the remaining 5% was explained by the loss of leg muscle mass. Medication intake was associated with an increased risk of decreasing power in women (OR = 1.87, <i>p</i> = 0.049), while a trend was shown in men (OR = 2.58; <i>p</i> = 0.052). Sedentary time showed a tendency only in women (OR = 1.73; <i>p</i> = 0.085).</p> Conclusion <p>Relative STS_power decreased progressively with aging, but differently in older men and women. This decline was primarily driven by reductions in specific STS_power, while the loss in leg muscle mass contributed to a lesser extent. Medication and sedentary time may influence power loss.</p>