Objective: The aim of our study was to determine the reliability and validity of the Trunk Impairment Scale (TIS) in patients with adult neuromuscular diseases (NMD). Materials and Methods: Sixty-six patients with NMD participated in the study. Patients were evaluated using the TIS, Manual Muscle Test (trunk, upper and lower extremities), Motor Function Measurement (MFM), Functional Independency Measurement (FIM), and the Rivermead Mobility Index (RMI). Results: Test-retest reliability was excellent for the TIS. The intra-class correlation coefficient score was 0.98. The Cronbach alpha value was 0.82. The TIS was found to be correlated with MFM (r=0.30), MFM trunk (r=0.31), trunk muscle strength (r=0.34), and lower extremity muscle strength (r=0.60, p<0.05) for construct validity. The TIS was found to be correlated with FIM (r=0.32, p<0.05) and RMI (r=0.39, p<0.05) for convergent validity. Conclusion: Our results confirm that the TIS is an easy, intra-rater reliable, and a valid instrument for the measurement of trunk performance in ambulatory patients with NMD. TIS is a measure that can be completed easily and in a short time in rehabilitation clinics.