Background Rectal cancer, accounting for a significant proportion of colorectal malignancies, presents unique challenges in treatment. Surgery remains the primary curative approach, but recurrence rates post-surgery poses challenges. While neoadjuvant chemoradiation has improved outcomes, the role of adjuvant chemotherapy is still debated. Herein, we aimed to clarify the efficacy of adjuvant chemotherapy in patients with confirmed pathological stage II rectal cancer. Methods In this retrospective single-center study, we investigated the role of adjuvant chemotherapy in 173 patients with biopsy-proven stage II rectal adenocarcinoma. Participants received neoadjuvant chemoradiation followed by open TME surgery, with or without adjuvant chemotherapy. The study was conducted at Sina Hospital between January 2014 and 2019, and analyzed overall survival (OS) and disease-free survival (DFS) outcomes. Propensity score matching (PSM) was used to adjust for potential confounders. Survival outcomes were assessed using Cox proportional hazards models, and sensitivity analysis was conducted using doubly robust estimation. Results Before matching, 173 patients showed significantly improved overall survival (HR:0.33, 95%C:0.22-0.50, p <
0.002) and disease-free survival (HR:0.41, 95%CI:0.28-0.61, p <
0.002) with adjuvant chemotherapy. Age ≥ 70 years was associated with poorer overall survival (HR:1.76, 95%CI:1.08-2.88, p = 0.02). After matching, in 100 patients (50 with chemotherapy, 50 without), adjuvant chemotherapy remained significantly beneficial for both overall and disease-free survival (p <
0.002), while age ≥ 70 years continued to negatively impact overall survival. Conclusion Our findings suggest that adjuvant chemotherapy provides benefits in terms of OS and DFS in stage II rectal cancer following neoadjuvant chemoradiation and TME surgery. Further prospective studies are warranted to confirm these results and optimize treatment strategies.