Background: Primary tumor sidedness (PTS) has been shown to be an independent prognostic factor for patients with metastatic colorectal cancer (CRC), with worse prognosis for right-sided tumors. There are limited data regarding the prognostic impact of PTS in stage III CRC. Objective: The main objective of this study was to analyze the prognostic impact of PTS in stage III CRC. Methods: A retrospective and uni-institutional cohort study was performed in a reference center in São Paulo-SP, Brazil. All consecutive patients with stage III CRC and treated with a 5-fluorouracil and oxaliplatin-based chemotherapy regimen (mFLOX regimen) from October 2007 to February 2013 were included. The primary outcome was probability of overall survival (OS) at 5 years stratified by PTS. Secondary outcomes were probability of disease-free survival (DFS) at 5 years stratified by PTS, and analysis of the prognostic impact of clinical and molecular biomarkers. The time-to-event variables were reported by the Kaplan Meier estimation method and prognostic factors associated with OS and DFS were assessed by Cox models. Results: 265 patients were included. Transverse colon tumors, multicentric tumors and undetermined primary subsite were later excluded, totaling 234 patients who were classified according to PTS: 95 on the right side (40.6%) and 139 on the left side (59.4%). Median age was 58 and 59 years for right- and left-sided tumors, respectively. For both groups, approximately 67% had stage IIIB. The median follow-up time was 66 months (range: 0-149). The 5-year OS probabilities for right-sided and left-sided tumors were 67% (95% CI: 58%-77%) and 82% (75%-89%), respectively. [Hazard ratio [HR]=2.02, 95% CI: 1.18–3.46; p=0.010). The 5-year probability of DFS for right-sided and left-sided tumors were 58% (49%-69%) and 65% (58%-74%), respectively. [HR=1.29, 0.84-1.97; p=0.248). Of the 234 patients, 61 patients (26%) had tissue samples available for DNA sequencing by NGS and immunohistochemical analysis. Of these, 12 patients (19.7%) had a RAS mutation, no patient had a BRAF mutation (V600E) and 6 patients (9.8%) had DNA repair enzyme deficiency (dMMR). No statistically significant difference was identified in the probability of OS and DFS at 5 years when comparing patients with right-sided and left-sided tumors when stratified by RAS mutation status and MMR status. Among patients with right-sided tumors, age >70 years (HR=4.11, 1.67-10.1; p=0.002), pN2 (HR=2.52, 1.23-5.17; p=0.012) and high-risk stage III according to the IDEA trial (HR=2.95, 1.35-6.44; p=0.002) were predictors of worse OS at 5 years. Among patients with left-sided tumors, poorly differentiated histology (HR=3.93, 1.44-10.7; p=0.007) and angiolymphatic invasion (HR=5.23, 1.54-17.8; p=0.008) were predictors of worse OS in 5 years. Conclusions: Data indicate a worse prognosis (inferior OS at 5 years) for resected right-sided stage III CRC treated with adjuvant mFLOX regimen.