Postoperative visits to the emergency department (ED) generate additional costs on health care systems and consume already limited resources. We sought to determine the rate and reasons why patients who underwent primary knee arthroplasty returned to the ED within 90 days of postoperative discharge, which could guide the development of preventative measures. We identified patients who underwent primary knee arthroplasty at 2 hospitals in Nova Scotia, Canada, between April 2021 and March 2022 and who had an ED visit within 90 days of postoperative discharge. We reviewed data for timing and reason for ED visits. Of 687 patients who had primary knee arthroplasty surgeries, 145 (21.1%) patients visited the ED within 90 days of postoperative discharge. Most visits occurred in the early postoperative period, predominantly for surgical reasons (94.7% within 7 days) and shifting toward medical reasons over time. Pain was the most common surgical reason for ED visits (15.9%), followed by swelling (9.7%), and hematoma drainage (9.7%). Understanding the reasons for postoperative ED visits after knee arthroplasty is important in developing strategies to reduce ED overcrowding and improve patient outcomes. Implementing targeted interventions, such as enhanced pain management and patient education, may help alleviate the burden on EDs, reduce costs, and improve overall health care delivery for patients undergoing knee arthroplasty.