The purpose of our study was to assess the test-retest reliability and concurrent validity of the 30 sec Sit to Stand test in a sample of adolescents. We recruited 30 male (58%) and 22 female (42%) participants (mean age = 15.77 years ± 0.46). Participants completed the 30-sec Sit to Stand and standing long jump tests on two occasions separated by 1 week. The rank order repeatability of the Sit to Stand test for the entire group was good (ICC = 0.84, 95% CI's [0.71 to 0.91]), systematic error was low (1.7 repetitions ± 2.9), and the typical error between tests was 2.1 repetitions (95% CI's 1.73 to 2.55). Sit to Stand results were significantly associated with lower body power (r = 0.55, p < 0.001). Our study demonstrated that the 30-sec Sit to Stand test has moderate-to-high test--retest reliability and acceptable concurrent validity in a population of adolescents.