Abstract: Colorectal cancer (CRC) is one of the most prevalent gastrointestinal tumors; however, its pathogenesis is poorly understood. Therapeutic strategies for CRC vary according to molecular typing. Immune checkpoint inhibitors (ICIs) have shown significant efficacy in patients with mismatch repair-deficient/microsatellite instability-high (dMMR/MSI-H) CRC. However, ICIs are less effective in patients with mismatch repair-proficient/microsatellite instability-low (pMMR/MSI-L). Intestinal flora have been found to enhance immunotherapy for CRC and ameliorate immunotherapy-related adverse effects by modulating immune responses through multiple mechanisms, particularly in patients with pMMR/MSI-L CRC. This article reviews the key role of intestinal flora in regulating immunotherapy for CRC, discusses the progress of related studies, and considers the clinical translational prospects of diet, probiotics, and fecal microbiota transplantation (FMT).