METHODS Large cohort study of volunteers. OBJECTIVE The purpose of this study was to investigate the relationship between the severity of knee osteoarthritis, assessed using the Kellgren-Lawrence (KL) grading scale, and spinopelvic sagittal alignment in older adult volunteers. BACKGROUND The relationship between spinopelvic alignment in the sagittal plane and knee osteoarthritis in the coronal plane is unclear. METHODS Volunteers over 50 years of age underwent radiographic analysis. Radiographic parameters including pelvic tilt (PT), pelvic incidence (PI), lumbar lordosis (LL), thoracic kyphosis, and sagittal vertical axis (SVA) were measured. The the three Scoliosis Research Society-Schwab sagittal modifiers (PT, SVA, PI-LL) were categorized and the KL grade was assessed. Differences in spinopelvic parameters and Oswestry Disability Index (ODI) scores among KL grades were evaluated. RESULTS A total of 396 volunteers (160 men, 236 women; mean age, 74.4 years) were analyzed. PI-LL and PT in KL4 were significantly higher compared to that in the other KL grades. However, there were no significant group differences in SVA. In women, but not in men, higher frequencies of the worst modifier grade (++) were observed for PI-LL and PT in the KL3 and KL4 groups compared to those for the other KL grades. In women, the ODI score in KL4 was worse compared to that in the other KL grades. CONCLUSIONS Individuals over 50 years of age with severe knee osteoarthritis had poor lumbo-pelvic sagittal alignment. Moreover, the progression severity of knee osteoarthritis had more impact onstronger relationship with lumbo-pelvic malalignment and disability-related low back pain in women than in men.