Introduction: Metastatic colorectal cancer (CRC) continues to present a significant clinical and public health burden, with metastasis accounting for the majority of CRC-related mortality. Recent evidence highlights the gut microbiota as an important modulator of not only CRC initiation but also metastatic progression, prognosis, and therapeutic response. However, the clinical significance and mechanistic roles of specific microbial alterations in metastatic CRC remain incompletely understood. Objective: To systematically review and synthesize cohort-based evidence regarding the association between gut microbiota composition and metastatic progression, risk, or prognosis in colorectal cancer patients. Methods: This systematic review adhered to PRISMA guidelines and involved a comprehensive search of PubMed, Embase, and Web of Science for cohort and observational studies published up to July 2025. Eligible studies included adult CRC patients, profiled the natural gut microbiota using sequencing, and compared microbial profiles by metastatic status or evaluated microbiota associations with metastatic outcomes. Data extraction and quality assessment were performed independently by two reviewers. Due to heterogeneity, a narrative synthesis approach was used. Result: Five cohort studies, published between 2022 and 2024, met inclusion criteria and included diverse populations from Europe and Asia. Across studies, metastatic CRC patients displayed significant shifts in gut microbiota compared to non-metastatic cases and healthy controls. Key findings included enrichment of Fusobacteria and pathogenic taxa (e.g., Bacteroides fragilis, Akkermansia muciniphila) in metastatic disease, alongside depletion of beneficial butyrate producing genera (Faecalibacterium prausnitzii, Blautia, Agathobacter). Fusobacteria enrichment was reproducibly linked to liver metastasis and poor progression-free survival, while higher abundance of butyrate-producers correlated with better prognosis, particularly in patients receiving immunotherapy. Considerable heterogeneity was observed due to confounding factors such as antibiotic use and baseline patient characteristics. Discussion: These findings indicate that metastatic CRC is associated with distinct and reproducible alterations in the gut microbiota, with Fusobacteria enrichment and loss of butyrate-producing commensals representing hallmark signatures. Such microbial shifts may contribute to tumor immune evasion, metastatic niche formation, and therapeutic resistance, while also holding potential as prognostic biomarkers. However, methodological differences, limited sample sizes, and variable adjustment for confounders across studies highlight the need for further validation in large, harmonized cohorts. Conclusion: Metastatic progression in CRC is characterized by significant alterations in gut microbiota composition, supporting the microbiome as both a prognostic biomarker and a potential therapeutic target. Future large-scale, longitudinal, and mechanistic studies are needed to confirm these findings, clarify causality, and pave the way for microbiota-based interventions in the management of metastatic CRC.