Background and Purpose: Few physical performance measurement tools are validated for acutely admitted older adults, and for this reason we aimed to examine the validity and responsiveness to change of the 30-second Chair-Stand Test (30s-CST) used to assess physical performance in older adults admitted to a short-stay unit in an emergency department. Methods: Construct validity of the 30s-CST, using 8 as a cutoff point for dependency in activities of daily living, was examined using 207 patients. Self-reported information on everyday activities was obtained by asking patients about need for help in bathing, dressing, cooking, cleaning, and shopping. Concurrent validity of the 30s-CST compared with the de Morton Mobility Index (DEMMI) on physical performance of acutely admitted older adults was examined with 156 patients. The analysis of concurrent validity included the entire DEMMI and 2 subsets of DEMMI: “DEMMI walking” and “DEMMI dynamic balance.” The responsiveness to change in the 30s-CST compared with DEMMI was examined with 117 patients. All patients were classified as having either low physical performance (30s-CST ≤8) or high physical performance (30s-CST >8); these groups were used in the analysis of validity and responsiveness to change. Results and Discussion: Regarding construct validity using 8 as a cutoff point, the study showed a significant difference between patients with low physical performance compared with patients with high physical performance. Moreover, a decrease in the 30s-CST was followed by an increase in the need for help with everyday activities. There was a significant association between the 30s-CST and DEMMI (r = 0.72); for every extra repetition in the 30s-CST, the DEMMI score increased by 4.9. There was a significant association between the 30s-CST and the 2 subsets “DEMMI walking” and “DEMMI dynamic balance”; yet, a pronounced floor effect was found in the subsets. The analysis demonstrated a very wide prediction interval, indicating that DEMMI has a better responsiveness to change than the 30s-CST, especially in older adults with low physical performance. However, the 30s-CST is easier and faster to use than DEMMI. Conclusion: This study found a significant difference in the patients' need for help with everyday activities when comparing low and high physical performance groups. The concurrent validity of the 30s-CST was acceptable in assessing physical performance in older adults at the time of admission; the 30s-CST is thus a tool that is easy to use in older adults with acute disease. In contrast, based on very wide prediction intervals, DEMMI demonstrated better responsiveness to change than the 30s-CST, especially in older adults with low physical performance.