Introduction: The Western Ontario Shoulder Instability Index (WOSI) is a self-administered questionnaire specifically used to determine the impact of shoulder instability on quality of life. The aim of this study was to translate the WOSI into European Portuguese and analyze its validity and reliability in a population with shoulder instability.Material and Methods: The WOSI was translated and culturally adapted from its original version into European Portuguese (WOSIPT). Internal consistency and test-retest analyses were conducted to determine the level of reliability of the scale. WOSI-PT, Quick-DASH, and SF-12 questionnaires were applied to 81 patients with symptomatic shoulder instability to assess validity, and reliability was tested by randomly selecting 50 patients within 72 hours using a test-retest design.Results: The reliability of the WOSI-PT was very high, with Cronbach´s alpha equal to 0.97 and an intraclass correlation coefficient of 0.98. Regarding the construct validity, the correlation between the WOSI-PT and QuickDASH was high and negative (-0.79). The correlations between WOSI-PT and SF-12 were positive, respectively, moderate with physical (0.66) and low with mental (0.34) health.Conclusion: WOSI-PT is a reliable and valid instrument for assessing the functional impact of shoulder joint instability on quality of life.