Knee extension limitation is a risk factor for knee osteoarthritis (OA) progression. However, the soft tissue stiffness involved in knee extension limitation remains to be determined. This study aimed to clarify the relationship between maximum knee extension angle and tissue stiffness in patients with knee OA using ultrasound shear wave elastography (uSWE). Women aged > 50 years with medial knee OA participated　in this study. We evaluated the maximum knee extension angle in the prone position using a goniometer at 1° increments. The shear wave velocity (SWV) in the prone position at 15° knee flexion of the following tissues was measured using uSWE:​ medial and lateral posterior capsule, medial collateral ligament, popliteus muscle, biceps femoris short head (middle and distal), and medial and lateral gastrocnemius (middle and proximal). We performed separate simple linear regression analyses with maximum knee extension angle as a dependent variable and the SWV of each tissue as an independent variable. A total of 66 participants were included in this study. The maximum knee extension angle was significantly positively associated with the SWV of medial posterior capsule (β = 0.31, p = 0.012). Conversely, the maximum knee extension angle was negatively associated with the SWV of the proximal medial gastrocnemius (β = -0.35, p < 0.01). There were no associations between other tissues and the maximum knee extension angle. Our results suggest that stiffness of the medial posterior capsule is associated with knee extension limitation in patients with Knee OA.