BACKGROUND: Among many self-reported outcome measures, Western Ontario and McMaster Universities Arthritis Index (WOMAC) is one of the commonly used indexes for osteoarthritis patients. Despite high prevalence of musculoskeletal problems in Nepal, there is no validated tool available in the local language. Thus, this study aimed to translate the English WOMAC® into Nepali, and validate it for use in the future. METHODS: Guidelines by Beaton et al were followed for translation and cross-cultural adaptation of the original WOMAC into Nepali language. Diagnosed cases of knee osteoarthritis (OA) attending the rheumatology outpatient department of National Center for Rheumatic Diseases, Kathmandu were enrolled in the study. These patients were interviewed with the Nepali version. Internal consistency was measured by Cronbach's alpha and test-retest reliability by intra-class correlation coefficient (ICC). Correlation between domains of WOMAC was tested with visual analog scale (VAS) and numerical rating scale (NRS) of pain and stiffness. RESULTS: The test-retest reliability was good with ICC of >0.75 for all domains and items. Internal consistency was acceptable with Cronbach's alpha scores of 0.852, 0.704 and 0.955 for pain, stiffness and physical function domains, respectively. Strong correlation was observed between WOMAC stiffness domain and VAS for stiffness and NRS for stiffness with rho (ρ) values of 0.750 and 0.759, respectively. Moderate correlation was seen between WOMAC pain and VAS for pain and NRS for pain with ρ of 0.658 and 0.584, respectively. CONCLUSIONS: Nepali WOMAC is a reliable and valid instrument for assessment of disease severity and its impact in Nepali-speaking patients with OA.