Background and Aims Coactivator-associated arginine methyltransferase 1 (CARM1) is involved in the regulation of RNA and protein processing. Recent studies have revealed its crucial role in tumorigenesis and cancer progression. However, the correlation between CARM1 expression, pan-cancer prognosis, and the immune microenvironment remains unclear. This study was conducted to investigate the relationship between CARM1 expression and pan-cancer prognosis, immune microenvironment, and its potential biological mechanisms using bioinformatics approaches.Methods Using the TCGA, GTEx, and HPA databases, the expression pattern of CARM1 in various cancers and its prognostic relevance were analyzed. The correlation between CARM1 expression and the tumor microenvironment, as well as immune checkpoint genes, were analyzed using TCGA data. The PPI network of CARM1-interacting genes was constructed using the STRING database. GSEA was performed to investigate the potential biological mechanisms of CARM1 in pan-cancer.Results Compared to normal tissues, CARM1 was upregulated in most tumor tissues, and its expression was associated with prognosis in multiple cancers. CARM1 expression was correlated with immune cell infiltration and immune checkpoint gene expression. Enrichment analysis suggested that CARM1 might alter the biological function of urinary system tumors by affecting ribosome biogenesis. Additionally, it may influence the biological functions of digestive system tumors by modulating arginine biosynthesis, DNA replication, and the cell cycle.Conclusion CARM1 is aberrantly expressed in tumor tissues and may serve as a prognostic biomarker for multiple cancers. It is associated with the tumor microenvironment and immune checkpoint gene expression in pan-cancer and may contribute to tumorigenesis through multiple biological pathways, making it a potential therapeutic target for pan-cancer treatment.