BACKGROUND: Blood loss is among the main complications of total knee arthroplasty (TKA) and oxidative stress, and hemolysis caused by reactive oxygen species are one of the causes of hemoglobin (Hb) drop. Ascorbic acid is a potent antioxidant that can protect against reactive oxygen species. In this study, we aim to explore the antioxidant effect of ascorbic acid on blood loss and patient-reported outcomes following outpatient TKA. METHODS: Patients scheduled for outpatient primary TKA were enrolled in this randomized, double-blind clinical trial and were assigned to 1 of the 2 groups. The patients in the ascorbic acid group received intravenous vitamin C perioperatively. Patients in the placebo group received only normal saline. We calculated the blood loss using the Hb drop. Patient-reported outcomes such as Oxford Knee Score, Western Ontario and McMaster Universities Osteoarthritis Index, Knee Injury and Osteoarthritis Outcome Score, and Forgotten Joint Score were used to evaluate the postoperative pain and function in the 6-month follow-up. RESULTS: The patients who have received ascorbic acid had lower Hb drop (g/dL) (1.30 ± 0.72 vs 1.91 ± 0.84, CONCLUSIONS: Our findings demonstrated that perioperative use of ascorbic acid can reduce blood loss by nearly 36% on the first postoperative day and should be considered as an effective blood-preserving agent in conjunction with tranexamic acid during TKA.