Background and objectives: Pain catastrophizing is associated with acute pain after total knee arthroplasty. However, the association between pain catastrophizing and acute pain after unicompartmental knee arthroplasty (UKA) remains unclear. Methods: We investigated the incidence of predicted high-pain and low-pain responders, based on a preoperative Pain Catastrophizing Scale score >20 or ≤20, respectively, and the acute postoperative pain course in both groups. Patients undergoing UKA were consecutively included in this prospective observational cohort study. Pain at rest and during walking (5 m walk test) was evaluated preoperatively, at 24 hours postoperatively, and on days 2-7 using a pain diary. Results: 125 patients were included, with 101 completing the pain diary. The incidence of predicted high-pain responders was 31% (95% CI 23% to 40%). The incidence of moderate to severe pain during walking at 24 hours postoperatively was 69% (95% CI 52% to 83%) in predicted high-pain responders and 66% (95% CI 55% to 76%) in predicted low-pain responders; OR 1.3 (95% CI 0.5 to 3.1). The incidence of moderate to severe pain at rest 24 hours postoperatively was 49% (95% CI 32% to 65%) in predicted high-pain responders and 28% (95% CI 19% to 39%) in predicted low-pain responders; OR 2.6 (95% CI 1.1 to 6.1; p=0.03). Pain catastrophizing was not associated with increased cumulated pain during walking on days 2-7. Conclusions: The incidence of predicted high-pain responders in UKA was slightly lower than reported in total knee arthroplasty. Additionally, preoperative pain catastrophizing was not associated with acute postoperative pain during walking.