Background/aim: Considering that there is no specifically designed scale to measure quality of life (QoL) and level of functioning among Turkish-speaking patients with thoracic outlet syndrome (TOS), the aim of this study was to adapt the Thoracic Outlet Syndrome Index (TOSI) to the Turkish language (TOSI-TR) and analyze its psychometric properties in patients with TOS. Materials and methods: Thirty-nine patients with symptomatic TOS were included in the study. The participants were assessed using the following measures: the QuickDASH, the Western Ontario Rotator Cuff Index (WORC), and a visual analog scale (VAS). The psychometric properties of the TOSI-TR were examined in terms of test–retest reliability, construct validity, convergent validity, and feasibility. Results: The ICC2,1 of the TOSI-TR was 0.949 (95% CI: 0.903–0.973). The Kaiser–Meyer–Olkin value was found to be 0.716 with a significant result for Bartlett’s sphericity test (p < 0.001). The TOSI-TR had a one-factor solution explaining 74.05% of the total variance. There was a significant correlation between TOSI-TR scores and QuickDASH (r = 0.762, p < 0.001), WORC (r =0.794, p < 0.001), and VAS (r = 0.663, p < 0.001) scores. The WORC and VAS scores were significant determinants of the TOSI-TR score, explaining 65.3% of the variance. There were no floor or ceiling effects. Conclusion: The TOSI-TR is a reliable, valid, and feasible questionnaire for the QoL and functional status in Turkish-speaking patients with TOS. We recommend this 15-item scale for optimal assessment in patients with TOS.