Abstract Background Use of conservative treatment modalities in osteoarthritis (OA) is suboptimal, which appears to be partly due to patients’ beliefs about treatments. The aim of this study was to develop a research instrument assessing patients’ beliefs about various treatment modalities of hip and knee OA: the ‘Treatment beliefs in OA (TOA) questionnaire’. Methods The item pool that was retrieved from interviews with patients and healthcare providers comprised beliefs regarding five treatment modalities: physical activity, pain medication, physiotherapy, injections and arthroplasty. After an extensive selection procedure, a draft questionnaire with 200 items was constructed. Descriptive analyses and exploratory factor analyses with oblique rotation were conducted for each treatment modality separately to decide upon the final questionnaire. Internal consistency and test-retest reliability were determined. Results The final questionnaire comprised 60 items. It was completed by 351 patients with knee or hip OA. Each of the five treatment modalities yielded a two factor solution with 37% to 51% explained variance and high face validity. Factor I included ‘positive treatment beliefs’ and factor II ‘negative treatment beliefs’. Internal consistency (Cronbach α’s from 0.72 to 0.87) and test-retest reliability (i.e. intraclass correlation coefficient from 0.66–0.88; standard error of measurement from 0.06–0.11) were satisfactory to good. Conclusions The TOA questionnaire is the first questionnaire assessing positive and negative treatment beliefs regarding five treatment modalities for knee and hip OA. The instrument will help to understand whether and to what extent treatment beliefs influence treatment choices.