BACKGROUND: Numerous studies have been conducted to manage anemia in surgical patients through iron supplementation as an alternative to blood transfusion. However, patients with locally advanced rectal cancer have often been excluded from these studies, due to their standard treatment involving neoadjuvant chemoradiotherapy. This study aims to evaluate the impact of intravenous versus oral iron supplementation on iron deficiency anemia in patients with rectal cancer receiving preoperative chemoradiotherapy. METHODS: This open-label, single-center, parallel, superiority, randomized trial includes patients with primary rectal cancer who are candidates for preoperative chemoradiotherapy and have confirmed iron-deficiency anemia. A total of 94 patients will be randomly assigned in a 1:1 ratio to receive either intravenous or oral iron supplementation. Stratification factors include age (> 70 vs. ≤ 70 years) and baseline serum hemoglobin levels (7-10 g/dL vs. 10-13 g/dL). The primary endpoint is the percentage of patients achieving normalized hemoglobin levels from the start of treatment to the day of admission for surgery. Secondary endpoints include changes in serum hemoglobin from baseline to postoperatively, changes in iron assay parameters, time needed to hemoglobin normalization, volume of blood transfusions required, and incidence of postoperative complications. DISCUSSION: This study is the first randomized controlled trial investigating the effect of iron supplementation in iron-deficient patients with rectal cancer undergoing neoadjuvant chemoradiotherapy. This trial is expected to provide evidence for the benefits of administering iron supplementation in patients with rectal cancer undergoing neoadjuvant chemoradiotherapy. TRIAL REGISTRATION: Clinical Research Information Service (CRIS) of Republic of Korea, KCT0009260, Registered on March 21, 2024.