Background and Objectives: The Western Ontario Shoulder Instability Index (WOSI) is a disease-specific self-administered questionnaire which is designed to measure health-related quality of life for patients with shoulder instability. The objective of this study was to translate and adapt the WOSI questionnaire for the Lithuanian-speaking population and investigate the psychometric properties of the Lithuanian version of the WOSI questionnaire (WOSI-LT): validity, reliability, and responsiveness. Materials and Methods: The WOSI scale was translated into Lithuanian using D. E. Beaton’s systematic and standardized guidelines for cross-cultural adaptation of patient-administered scales. Subsequently, the psychometric properties of the Lithuanian version of the scale (WOSI-LT) were investigated. The study involved 40 patients who reported shoulder instability and underwent surgical treatment. All patients completed the WOSI-LT, QuickDASH, and SF-12 scales. A subset of 10 patients was selected for the reproducibility and responsiveness evaluation. Based on the obtained data, the reliability, validity, and responsiveness of WOSI-LT were examined using statistical analysis methods. Results: The Lithuanian adaptation of the WOSI questionnaire exhibited a high degree of internal consistency, evidenced by a Cronbach’s alpha of 0.93. Its reproducibility was commendable with an intraclass correlation coefficient (ICC) value of 0.90. When assessing correlations, WOSI-LT demonstrated a stronger relationship with QuickDASH (r = 0.64) than with SF-12 (physical component score (PCS) 0.61, mental component score (MCS) 0.33). Six months post-operation, the responsiveness of the WOSI-LT was particularly notable, with a standardized response mean (SRM) of 0.91, the highest among the three scales. Furthermore, no floor or ceiling effects were identified in the scores of the Lithuanian WOSI. Conclusions: WOSI-LT is a valid, reliable, and responsive questionnaire that correlates excellently with the original English version of the scale. This scale can be used in Lithuanian medical institutions to assess the severity of patients’ shoulder instability and evaluate their progress during treatment.