One way to measure the effectiveness of a specific treatment is to utilize measurements designed specifically for the disorder. Western Ontario Shoulder Instability Index (WOSI) is a subjective self-report scale indicating the latest condition of the patients with shoulder instability. The objective is to study the cultural adaptation, validity, and reliability of WOSI in Turkish population with shoulder disability. First, WOSI was translated and culturally adapted from English into Turkish. Afterward, in order to determine the level of reliability, internal consistency and test-retest analyses were conducted. Reliability (test-retest) analyses were conducted by means of retest 72 h later with a sub-group of 30 patients. Construct validity of the WOSI was checked through convergent validity with Disabilities of Arm, Shoulder and Hand Scale, Rowe Score Questionnaire, Oxford Shoulder Instability Questionnaire, and Western Ontario Rotator Cuff Index by 60 patients with shoulder instability. The Turkish version of the questionnaire displayed high internal consistency (0.77-0.91) with a Cronbach's Alpha value of 0.91. As for the test-retest reliability, the ICC value was found to be high (95% CI 0.97). Floor and ceiling effects (15%) were observed neither in sub-parameters (0-4.9%) nor in total score (0%). WOSI total score was found to have a negative good correlation with the Rowe Score (r = -0.57) and a very good-excellent correlation with other questionnaires (r = 0.67-0.89). The Turkish version of WOSI is a valid and reliable scale for use in studies to evaluate the final condition of the patients with shoulder disabilities.