Background and Objectives Previous studies have concluded that colorectal cancer patients with deficient mismatch repair (dMMR) usually have a good prognosis. However, some studies have suggested that the prognosis of rectal cancer patients with dMMR appears to be worse. Our aim was to investigate chemoradiotherapy resistance in dMMR rectal tumors. Methods A retrospective study of 217 patients with locally advanced rectal adenocarcinoma treated with chemoradiotherapy and total mesorectal excision surgery was conducted using immunohistochemistry to determine MMR status and propensity score matching models to reduce potential confounders. Kaplan-Meier analysis, log-rank test, and Cox regression models were used to assess overall survival (OS) and disease-free survival (DFS) in patient subgroups. Results The 3-year DFS rates were 77.1% and 56.7% in the pMMR and dMMR groups, respectively. The pMMR group had significantly better DFS than the dMMR group (hazard ratio [HR], 2.07; 95% confidence interval [CI], 1.10-3.91; p = 0.019). However, there was no significant difference in OS between the two groups (45.7 [interquartile range, IQR], 39.3-72.1] vs. 47.5 [IQR, 29.5-72.1]) (HR, 1.39; 95% CI, 0.70-2.77; p = 0.35). Neither OS nor DFS was significantly different between the neoadjuvant chemoradiotherapy and postoperative chemoradiotherapy groups. Conclusion Locally advanced dMMR rectal adenocarcinoma exhibits greater chemoradiotherapy resistance than pMMR.