Background: Adjuvant chemotherapy is the current standard treatment for stage III colorectal cancer after curative resection. However, the prognosis of stage III colorectal cancer is still poor even after curative resection and adjuvant chemotherapy. Recently, several observational studies suggested the anti-tumor effect of aspirin for advanced colorectal cancer. The main mechanism of the anti-tumor effect by aspirin may be to suppress cyclooxygenase activity in the arachidonic acid cascade and to inhibit the production of prostaglandins involved in tumor growth. So far, aspirin showed a prolongation of survival for colorectal cancer in several retrospective studies. However, in these studies, aspirin was given not to be evaluated the effect on prognosis of colorectal cancer in adjuvant setting but to prevent cardiovascular event. In addition, baseline patient characteristics were imbalanced between aspirin group and non-aspirin group and both dosage amount and dosing period of aspirin were different among patients. Methods: We planned a randomized double-blind placebo-controlled phase III trial commenced in Japan in March 2018 to confirm the superiority of aspirin in terms of disease-free survival (DFS) over placebo for stage III colorectal cancer patients after curative resection. Patients receive aspirin (100 mg/day) or placebo for 3 years with the standard adjuvant chemotherapy of mFOLFOX6, CAPOX or capecitabine until relapse or unacceptable toxicities. The primary endpoint is DFS and the secondary endpoints are overall survival, relapse-free survival, relative dose intensity, adverse events, and serious adverse events. We assumed the 3-year DFS of aspirin arm as 74% based on two previous trials conducted by JCOG and expected a 6% increase in the 3-year DFS with aspirin adding to standard adjuvant chemotherapy after curative surgery. A total of 880 patients will be accrued from 20 Japanese institutions within 3 years, and 47 patients were enrolled as of Jan 31, 2019. Both aspirin and placebo are provided by Bayer Yakuhin Ltd. This trial has been registered at Japan Registry of Clinical Trials as jRCTs031180009 (https://jrct.niph.go.jp/detail/589). Clinical trial information: jRCTs031180009.