Fecal microbiome culturomics of a cancer patient treated with immune checkpoint inhibitors led to the identification of a Gram-negative, rod-shaped, obligate anaerobic, non-motile, non-spore-forming bacterium, designated strain kh20^T, which was phylogenetically assigned to the genus Alistipes. Strain kh20^T demonstrated a 98.61% 16S rRNA sequence similarity with A.shahii WAL 8301^T. The bacteria cells generated catalase but no oxidase. Iso-C_15:0 (26.6%), anteiso-C_15:0 (19.9%), and iso-C_17:0 (17.2%) were the major cellular fatty acids identified in its composition. The G+C content of its genome was 57.2%. Strain kh20^T showed significantly low values for DNA-DNA Hybridation (DDH ≤ 33.70%) and Average Nucleotide Identity (ANI ≤ 86.35%) compared with other Alistipes species. Based on these findings, we concluded that strain kh20^T represented a novel bacterium, and we proposed the name Alistipes montrealensis sp. nov. (CECT 30384 and CSUR Q6005).