Recent studies suggest that patients with lower extremity osteoarthritis may have altered spinal posture. We aimed to investigate age and body mass index-adjusted sagittal spinal alignment and mobility and their relation to physical function in women over 40 years of age with and without mild-to-moderate knee osteoarthritis (KOA). Thirty-two women with unilateral mild-to-moderate KOA and thirty-two asymptomatic women were included. A skin-surface device was used to assess sagittal alignment and mobility of the thoracic, lumbar, and sacral regions and trunk inclination angle. Physical function was assessed using the Timed Up and Go test. Analysis of covariance was used to compare groups and correlation coefficients were calculated separately for two groups. Women with KOA had higher thoracic kyphosis and lumbar lordosis compared to asymptomatic women (p < 0.05). The mean differences were 6.60 (%95 Confidence Interval 1.38;​11.82) and −5.63 (−10.06;​−1.20) for thoracic kyphosis and lumbar lordosis, respectively. Physical function score was moderately correlated with trunk inclination angle and lumbar, sacral, and trunk inclination mobility in asymptomatic women (r = 0.400, −0.504, −0.602, and −0.681, p < 0.05), but a significant correlation was not found in women with KOA (p > 0.05). In conclusion, women with KOA had altered spinal alignment. Spinal alignment and mobility were related to physical function in asymptomatic women over 40 years of age. Addressing sagittal spinal alignment in the clinical management of KOA may provide valuable data, especially for preventing possible spinal disorders.