Background: Adjuvant chemotherapy for stage II colorectal cancer (CRC) still remains controversial. Although the NCCN and ESMO guidelines recommend adjuvant chemotherapy for patients with “high-risk features,” the survival benefit has not been confirmed. We reviewed the evidence levels for prognostic values of risk factors, because lack of their robustness is a major source of uncertainty regarding the optimal indication of adjuvant chemotherapy. Consequently, on top of the T-stage, three pathological factors—perineural invasion (Pn), tumor budding (BD), and desmoplastic reaction (DR)—were selected as robust risk factors of recurrence. Among the conventional factors, the prognostic value of Pn had been well validated in a multicenter study conducted by the Japanese Society for Cancer of the Colon and Rectum (JSCCR; Am J Surg Path 2013), but others were deemed suboptimal in terms of the prognostic value. BD and DR are novel tumor- and stroma-factors, respectively, associated with cancer microenvironment at the tumor front. According to the JSCCR and ITBCC 2016 criteria, tumors are graded as BD1, BD2, or BD3. The DR heterogeneity is categorized into Mature, Intermediate, and Immature patterns based on site-specific products of cancer-associated fibroblasts—keloid-like collagen and myxoid stroma. According to a recent prospective multicenter study, BD and DR characterization represent a higher level of prognostic value than other conventional factors (SACURA trial; J Clin Oncol 2019, Br J Cancer 2021). Based on the four selected risk factors, we can exclude the patient group with favorable prognosis (i.e., > 90% of 5-year RFS), which accounts for approximately 40% of the total population, resulting in enabling us to identify the concentrated population of high risk of developing recurrence. Methods: The Japan Clinical Oncology Group (JCOG) launched a randomized controlled phase III trial to evaluate the superiority of adjuvant chemotherapy in terms of relapse-free survival (RFS) over observation only in stage II CRC patients aged 20–80 years having one or more of the following risk factors: pathological T4, Pn, BD3, and non-Mature DR. Patients are randomised, in a 1:1:1 ratio, to [A] observation, [B] capecitabine monotherapy for 6 months, or [C] capecitabine and oxaliplatin (CAPOX) for 3 months. A total of 1680 patients will be accrued from 54 Japanese institutions assuming 3-year RFS with [A] to be 82% and expected 5% increase in 3-year RFS for [B] and [C] with one-sided alpha of 2.5% and power of 80% for each pair comparison. Patient enrollment was started in January 2020 and 170 patients have been enrolled until January 2021. This trial has been registered at Japan Registry of Clinical Trials as jRCTs031190186. Clinical trial information: jRCTs031190186.