Abstract Background : Diabetes mellitus (DM) has been reported to be an independent risk factor for the development and severity of knee osteoarthritis (OA). Additionally, patients with DM are often obese or exercise insufficiently, and patients with knee OA and DM may have decreased physical activity at both light and moderate-to-vigorous intensity. To date, the effects of DM on physical activity in patients with knee OA have not been reported. We measured physical activity levels by intensity in patients with moderate-to-severe knee OA to confirm the effects of DM on physical activity levels in these patients. Methods : We evaluated 183 patients with moderate-to-severe knee OA (28 men, 155 women) diagnosed with knee OA by radiography and scheduled for total knee arthroplasty. We evaluated knee function (knee flexion/extension range-of-motion, knee-extension strength, knee pain), performance-based physical function (Timed Up-and-Go [TUG] test), and objectively measured physical activity. The effects of DM on physical activity were evaluated using hierarchical multiple regression analysis. Results : Fewer daily average step counts were significantly associated with DM (β = -0.200; p = 0.006) and longer TUG time (β = -0.196; p = 0.014). Shorter light-intensity physical activity (LPA) time was significantly associated with DM (β = -0.216; p = 0.004) and longer TUG time (β = -0.208; p = 0.011). Shorter moderate-to-vigorous-intensity physical activity (MVPA) time was significantly associated with lower contralateral knee-extension strength (β = 0.187; p = 0.032). Conclusions : DM, mobility, and balance ability affected step counts and LPA time, and contralateral knee-extension strength affected MVPA time in patients with moderate-to-severe knee OA. Our results imply that education on the benefits of exercise is important in patients with moderate-to-severe knee OA, especially those with DM, and there is a need for therapies to improve balance and strengthen the knee-extension muscles in this patient population, in order to increase their physical activity levels.