BACKGROUND: Unused medications in homes pose significant health, economic, and environmental risks. Patients are the medications end users and their knowledge, attitude, and practices (KAP) play an important role towards medication use, unuse and wastage. Thus, a valid instrument to reliably measure patients' KAP towards unused medications in homes may help manage the associated risks. OBJECTIVE: To develop, translate, and validate a questionnaire for the assessment patients' KAP towards unused medications in Qatar homes (i.e., QUM-Qatar). SETTING: This cross-sectional validation study was conducted among randomly selected outpatients visiting various public and private pharmacies in Qatar between September 2019 and February 2020. METHOD: Nine experts in the field of pharmacy practice with Qatar contextual background established the content validity of the instrument. The validity was quantified using content validity index (CVI). Furthermore, construct validity was performed using principal component analysis (PCA), whereas internal consistency reliability of items was determined using Cronbach's alpha. Statistical analyses were performed using STATA 15 statistical software. MAIN OUTCOME MEASURE: The psychometric properties of the QUM-Qatar assessment instrument. RESULTS: An English/Arabic questionnaire was developed and validated. Content validity in the form of scale-level-CVI (S-CVI)/Average and S-CVI/UA was 0.88 and 0.84, respectively, suggesting adequate relevant content of the questionnaire. Variation explained by the multivariate model was 85.0% for knowledge, 94.8% for attitude, and 89.8% for practice. Cronbach's alpha coefficients were 0.68, 0.82, and 0.84, for knowledge, attitude, and practice domains, respectively. From the psychometric results obtained, the questionnaire's validity and reliability were attained. CONCLUSION: The QUM-Qatar instrument has acceptable psychometric properties and has the potential for future use in research and practice to assess KAP towards unused medications in Qatar and elsewhere. It may consequently help in improving medication use and mitigating the negative health, economic, and environmental impacts of unused medications.Impacts on practice.•There is now a valid and reliable English/Arabic language questionnaire to assess patients' KAP towards unused medications.•Policymakers can utilize this questionnaire to develop evidence-based policies and strategies for managing unused medications and their disposal.•To improve medication use review, rational use of medicines, and adherence, it is necessary to consider patient-reported outcomes that may eventually reflect on saving health, economic resources, and environment.