Abstract Objective AT-rich interaction domain 1A (ARID1A) mutant tumors show active anti-tumor immune response, which is the potential indication of immunotherapy. However, the relationship between the heterogeneous ARID1A expression and the immune response and immunotherapy efficacy in colorectal cancer (CRC) is still unclear. Methods We collected 1113 cases of patients with stage I-IV CRC who underwent primary resection at Harbin Medical University Cancer Hospital. ARID1A expression in CRC tissues was assessed via immunohistochemistry (IHC). CD8, CD163 and FOXP3 were stained by IHC to identify the immune landscape. Clinicopathological features of patients were compared using statistical tests like the Wilcoxon-Mann–Whitney test or χ2 tests. Kaplan–Meier survival analysis with log-rank tests were employed. Results Heterogeneous ARID1A expression was categorized into integrity expression, complete expression deficiency (cd-ARID1A), partial expression deficiency (pd-ARID1A), and clonal expression deficiency (cld-ARID1A). ARID1A-deficient expression was significant association with dMMR (P value