The objective of this study was to cross-culturally adapt and validate the Portuguese version of the Knee Outcome Survey-Activities of Daily Living Scale (KOS-ADLS). This version was obtained with forward/backward translations, consensus panels and pre-testing. The Portuguese KOS-ADLS and Medical Outcomes Study, 36-item Short Form (SF-36) questionnaires, visual analogue scales (VAS) of pain, disability and discomfort, and a form for patient's characteristics were administered to 168 subjects with knee osteoarthritis (OA). Reliability was acceptable (Cronbach's alpha = 0.91; ICC = 0.97). There were significant correlations with SF-36 physical component subscales, all VAS, and duration of knee OA. The subjects with bilateral knee OA and that need walking aids obtained lower scores (p < 0.001). No floor/ceiling effects were detected. Responsiveness to physical therapy was showed (standardized effect size = 0.62; standardized response mean = 1.02). The Portuguese KOS-ADLS evidenced acceptable reliability, validity, floor/ceiling effects, and responsiveness.