BACKGROUND The present study was performed to determine the potential risk factors for postoperative knee stiffness in patients with anteromedial knee osteoarthritis undergoing unicompartmental knee arthroplasty with cemented prostheses. MATERIAL AND METHODS This retrospective cohort study evaluated patients with anteromedial knee osteoarthritis who underwent medial unicompartmental knee arthroplasty at our hospital between May 2017 and May 2020. The patients were divided into 2 groups according to their prognosis: those who experienced knee stiffness after undergoing unicompartmental knee arthroplasty and those who did not. The factors associated with stiffness after UKA were identified using univariate analysis. Frequencies are used to express categorical variables, while mean±SD is used to express continuous variables. The t test and chi-square test were used. A multivariate logistic regression model was built to identify the risk factors for postoperative stiffness. RESULTS We included 590 knees in the study after unicompartmental knee arthroplasty. The overall incidence of postoperative stiffness in unicompartmental knee arthroplasty surgery was 10.17%. In terms of the radiological measurements, varus deformity (70.34% vs 29.66%) and tibial component posterior slope angle (4.8±2.0 vs 4.6±2.0, P<0.001) were significantly differences between the 2 groups. Four independent risk factors for stiffness after unicompartmental knee arthroplasty were identified: age (95% CI, 1.022-1.048), varus deformity (95% CI, 1.186-1.192), tibial component posterior slope angle (95% CI, 0.550-0.870), and preoperative maximum flexion (95% CI, 0.896-0.923). CONCLUSIONS The overall incidence of postoperative knee stiffness in patients with anteromedial knee osteoarthritis undergoing unicompartmental knee arthroplasty with cemented prostheses was 10.17%, which was at a moderate level compared to patients with other diseases undergoing unicompartmental knee arthroplasty. Four independent risk factors were identified: age, varus deformity, preoperative maximum flexion, and tibial component posterior slope angle. Awareness these risk factors might help surgeons prevent the occurrence of postoperative knee stiffness in patients with UKA.