The aim of this study was to develop and to validate a Bengali version of the Western Ontario and McMaster Osteoarthritis (WOMAC) index in Bangladesh. The WOMAC was translated into the local language of Bangladesh (Bengali) and adapted in the local sociocultural context, following the standard guidelines by Beaton et al. Content validity of the preliminary Bengali version was assessed by using the index of content validity (ICV) and floor and ceiling effects. Patients were assessed at the Department of Rheumatology of Bangabandhu Sheikh Mujib Medical University and were diagnosed to have knee OA by American College of Rheumatology criteria and recruited according to the requirements of the validation study. Convergent and divergent validity were measured by comparing with Health Assessment Questionnaire (HAQ) and the Short Form-36 (SF-36), and internal consistency was assessed using Cronbach's alpha coefficient. The questionnaire was readministered to 40 patients within a week for assessing reliability by using intra-class correlation coefficient (ICC) and Spearman's rank correlation coefficient. In addition, factor analysis of Bengali WOMAC questionnaire was performed to examine the number of factors influencing a common set of items. A Bengali version was developed with changes in three items to suit local practices. The ICV of the content validity was 1 for all items. The Bengali WOMAC had similar construct validity when compared to the HAQ (ρ 0.74, n = 70) and SF-36 bodily pain and physical functioning. It had dissimilar construct validity to SF-36 mental health domain except WOMAC pain. Factor analysis revealed five factors with eigenvalues of more than 1.0. Cronbach's alpha and ICC exceeded 0.7 in all domains. In the test-retest reliability testing, Spearman's ρ for all items exceeded 0.4 (n = 40). This study has demonstrated that the Bengali version of WOMAC is a valid tool for assessing quality of life of patients with knee osteoarthritis in Bangladesh and is reliable.