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Colorectal cancer (CRC) is the leading cause of cancer-related deaths worldwide. Fibromodulin (FMOD) is the main proteoglycan that contributes to extracellular matrix (ECM) remodeling by binding to matrix molecules, thereby playing an essential role in tumor growth and metastasis. There are still no useful drugs that target FMOD for CRC treatment in clinics. Here, we first used public whole-genome expression datasets to analyze the expression level of FMOD in CRC and found that FMOD was upregulated in CRC and associated with poor patient prognosis. We then used the Ph.D.-12 phage display peptide library to obtain a novel FMOD antagonist peptide, named RP4, and tested its anti-cancer effects of RP4 in vitro and in vivo. These results showed that RP4 inhibited CRC cell growth and metastasis, and promoted apoptosis both in vitro and in vivo by binding to FMOD. In addition, RP4 treatment affected the CRC-associated immune microenvironment in a tumor model by promoting cytotoxic CD8+ T and NKT (natural killer T) cells and inhibiting CD25+ Foxp3+ Treg cells. Mechanistically, RP4 exerted anti-tumor effects by blocking the Akt and Wnt/β-catenin signaling pathways. This study implies that FMOD is a potential target for CRC treatment, and the novel FMOD antagonist peptide RP4 can be developed as a clinical drug for CRC treatment. [ABSTRACT FROM AUTHOR]