Background: AT-rich interactive domain 1A (ARID1A) is a chromatin regulator mutated in human cancers, frequently resulting in truncation and loss of expression of this protein. ARID1A recruits MSH2 during DNA replication to perform mismatch-repair. ARID1A deficiency has been shown to increase mutational load and immune activation in preclinical models (Shen J, Nat Med 2018) but its role in colorectal cancer (CRC) patients (pts) is being explored. Methods: The DNA sequencing and gene expression profiling of microsatellite-stable (MSS) CRC pts were extracted from TCGA and MD Anderson Cancer Center databases. The expression levels were normalized according to the mean values of each dataset. The mutational burden and expression signatures for IFN-γ and various immune markers were compared according to the ARID1A mutational status. Results: Among 417 pts with MSS CRC, 28 pts (6.7%) had a non-silent mutation in ARID1A. Out of the 58 genes most commonly mutated in CRC, non-silent mutation in ARID1A had the strongest association with the frame-shift mutation rate in MSS cases (8-fold increase, p< .001). ARID1A mutation had also a strong correlation with the IFN-γ expression signature in MSS CRC (Δz score +1.91, p< .001). Compared with ARID1A wild-type pts, higher expression signatures for cytotoxic T cell function, NK cells, and immune checkpoints were observed in MSS ARID1A mutated cases. ARID1A mutant cases showed higher expressions of various immune checkpoint genes (CD274, CTLA4, HAVCR2, IDO1, LAG3, PDCD1, and PDCD1LG2) compared to wild-type cases (all p < .05). All findings were observed independently in both datasets. Conclusions: In MSS CRC, ARID1A mutation is associated with a high expression of IFN-γ pathway and immune signatures (such as cytotoxic T cell function and immune checkpoint markers). The immunogenicity of ARID1A mutant cases is likely due to the increased level of neoantigens resulting from the increased rate of frame-shift mutations. Tumors with ARID1A mutation may be more susceptible to immune therapy-based treatment strategies and should likely be recognized as a unique molecular subgroup in future immune therapy trials.