Background: Patients with moderate-to-severe knee osteoarthritis (OA) or diabetes mellitus (DM) have lower physical activity levels. Whether the presence of both knee OA and DM further decreases physical activity is unclear. We investigated the differences in physical activity between knee OA patients with and without DM, and evaluated the association between physical activity and DM in knee OA patients.Methods: A total of 183 patients (mean age 74.9 ± 6.4 years) with moderate-to-severe knee OA underwent evaluation of knee flexion/extension range-of-motion, knee-extension muscle strength, knee pain, Timed Up-and-Go (TUG), and physical activity using an accelerometer.Results: The average daily step count was significantly lower (p < 0.001), and light-intensity physical activity (LPA) (p < 0.001) and moderate-to-vigorous-intensity physical activity (MVPA) (p = 0.006) times were significantly shorter in knee OA patients with DM than in those without DM. After adjusting for age, sex, and body mass index, a lower average daily step count and a shorter LPA time significantly correlated with DM (β = -0.200, p = 0.006; β = -0.216, p = 0.004, respectively) and a longer TUG test time (β = -0.196, p = 0.014; β = -0.208, p = 0.011, respectively). A shorter MVPA time significantly correlated with contralateral lower knee-extension muscle strength (β = 0.187, p = 0.032).Conclusions: Knee OA patients with DM had significantly lower physical activity levels than those without DM. Furthermore, the presence of DM correlated with a lower step count and a shorter LPA time in patients with knee OA.