Purpose: To investigate the validity and reliability of the Turkish version of the Ottawa Sitting Scale (OSS-T) in patients with acute stroke. Materials and methods: The Berg Balance Scale (BBS) and Trunk Impairment Scale (TIS) were used to determine the validity of the OSS-T. The OSS-T was re-applied by the same rater after an interval of 7 days to determine the reliability. To test inter-rater reliability, the evaluation was repeated by a second rater 1 day after the first evaluation. Reliability was quantified using intraclass correlation coefficients (ICC), and validity was assessed by correlating the OSS-T scores with the results of the other measures. Results: The ICC of the total OSS-T score for inter-rater reliability was 0.996 and for intra-rater reliability, it was 0.951. The Cronbach’s α coefficient used to determine internal consistency was 0.980, which indicates excellent reliability. A strong positive correlation was found between OSS-T and TIS (rho = 0.861, p < 0.001), and between OSS-T and BBS (rho = 0.875, p < 0.001). An evident 2-factor structure was shown by the results of the factor analysis. Conclusions: The results of this study indicated that the OSS-T has strong measurement properties, making it a valid and reliable tool for research and clinical practice in patients with acute stroke.IMPLICATIONS FOR REHABILITATION Independent sitting function is an important indicator of functional recovery and discharge from hospital. Unlike other sitting balance tools, the Ottawa Sitting Scale can even categorize patients with low balance reserve by applying all the items with and without foot support. The Turkish version of the Ottawa Sitting Scale is a valid and reliable tool to evaluate sitting balance in patients with acute stroke.