Prolonged sedentary behavior (SB) may affect clinical improvements following total knee arthroplasty (TKA). We aim to assess preoperative SB effect on improvements in clinical outcomes after TKA. We hypothesized that patients with high proportion of preoperative SB have small improvements in clinical outcomes after TKA. We recruited 115 adults with knee osteoarthritis (OA), planning to undergo TKA. Knee-specific functional outcomes were assessed using the new knee society score (KSS), including total score, symptoms, patient satisfaction, patient expectations, and functional activities. Gait function was assessed using the timed up and go (TUG) test. SB was measured using a triaxial accelerometer. Multiple linear regression analysis was performed to investigate preoperative SB effect on improvements in clinical outcomes 6 months postoperatively by adjusting for several factors as potential confounders. Of 115 patients, 82 were included. High proportion of preoperative SB had a significant negative effect on recovery at 6 months in the new KSS total score [β, -0.83, 95% confidence interval (CI), -1.53 to -0.12; p = 0.02], symptoms (-0.15, -0.28 to -0.02; p = 0.03), patient satisfaction (-0.22, -0.42 to -0.02; p = 0.03), and functional activities (-0.40, -0.76 to -0.04; p = 0.03) after adjusting for potential confounders. A high proportion of preoperative SB was a risk factor for reduced improvements in knee-specific functional outcomes after TKA. To prevent poor functional recovery after TKA, it is essential to understand the preoperative lifestyle factors and intervene proactively during acute postoperative phase in patients with high SB proportion.Key Points• Preoperative high proportion of SB had a negative effect on improvement in new KSS total scores, symptoms, patient satisfaction, and functional activities.• When considering clinical outcomes after TKA, clinicians should closely monitor patients with high proportions of preoperative SB to prevent poor functional recovery.