This study tested the concurrent and construct validity of smartphone accelerometry measurement of sit-to-stand (STS) performance in individuals aged 65-89 years. Normal and fast STS times were recorded by smartphone accelerometer, force plate, and video motion systems concurrently, and isokinetic knee extension power and STS whole-body power were obtained. Normal and fast speed STS times from a smartphone accelerometer agreed closely with force plate and motion system methods (mean difference = 0.04 s). Normal and fast STS times were inversely related to isokinetic knee extension power (r = -.93, p < .001 and r = -.82, p < .001, respectively) and STS whole-body power (r = -.76, p < .001 and r = -.70, p < .001, respectively). The STS time obtained from a smartphone accelerometer was equivalent to the established, precise measures of STS time and was related to lower-extremity power, making it a potentially useful metric of lower-extremity function.