Intratumoural microbiota, including bacteria, fungi, and viruses, play a critical role in cancer development and therapy. Recent studies have shown that previously considered sterile tumour tissues host diverse microorganisms, which are closely linked to oncogenesis. These microorganisms can colonize tumour tissues through mucosal invasion, adjacent tissue migration, and hematogenic invasion, influencing tumour biology and the tumour microenvironment (TME). Mechanistic studies suggest that intratumoural microbiota may promote tumour initiation and progression by inducing genomic instability, affecting epigenetic modifications, promoting inflammation, avoiding immune destruction, regulating metabolism, and activating invasion and metastasis. New methods for early cancer diagnosis and prognosis are being explored, and interventions targeting intratumoural microbiota show potential in antitumour therapy, particularly immunotherapy. This review summarizes the concept, development history, potential sources, heterogeneity, and carcinogenic mechanisms of intratumoural microorganisms, explores their role in tumour prognosis, and discusses current antitumour treatment regimens targeting intratumoural microorganisms. The review also highlights the potential and challenges in this field. The intratumoural microbiota has diverse origins, including mucosal barrier invasion, adjacent tissue invasion, and hematogenic invasion. The structure and abundance of the intratumoural microbial population vary across different cancers, with distinct microbial communities in various tumour types. The role of intratumoural microbiota in cancer development is multifaceted, involving genome instability and mutation, epigenetic modification, chronic inflammation, immune evasion, metabolic regulation, and activation of invasion and metastasis. The review also discusses the potential of intratumoural microbiota in cancer prognosis and therapy, as well as the challenges in this field. The findings suggest that intratumoural microbiota may directly or indirectly regulate host epigenetic modifications, including DNA modification, histone modification, RNA modification, and non-coding RNA. However, the molecular mechanisms underlying these changes need further investigation. Chronic inflammation is closely associated with most cancer types, and intratumoural microbiota can activate inflammatory signalling pathways, promoting tumour progression. Immune evasion is another key mechanism, with intratumoural microbiota promoting an immunosuppressive microenvironment and immune cell inactivation. Metabolic regulation is also a critical aspect, with microbiota altering human metabolism and contributing to various metabolic diseases and cancers. The review highlights the potential of intratumoural microbiota as a therapeutic target for supporting immunotherapy, with their effects being context-dependent and requiring further clarification.Intratumoural microbiota, including bacteria, fungi, and viruses, play a critical role in cancer development and therapy. Recent studies have shown that previously considered sterile tumour tissues host diverse microorganisms, which are closely linked to oncogenesis. These microorganisms can colonize tumour tissues through mucosal invasion, adjacent tissue migration, and hematogenic invasion, influencing tumour biology and the tumour microenvironment (TME). Mechanistic studies suggest that intratumoural microbiota may promote tumour initiation and progression by inducing genomic instability, affecting epigenetic modifications, promoting inflammation, avoiding immune destruction, regulating metabolism, and activating invasion and metastasis. New methods for early cancer diagnosis and prognosis are being explored, and interventions targeting intratumoural microbiota show potential in antitumour therapy, particularly immunotherapy. This review summarizes the concept, development history, potential sources, heterogeneity, and carcinogenic mechanisms of intratumoural microorganisms, explores their role in tumour prognosis, and discusses current antitumour treatment regimens targeting intratumoural microorganisms. The review also highlights the potential and challenges in this field. The intratumoural microbiota has diverse origins, including mucosal barrier invasion, adjacent tissue invasion, and hematogenic invasion. The structure and abundance of the intratumoural microbial population vary across different cancers, with distinct microbial communities in various tumour types. The role of intratumoural microbiota in cancer development is multifaceted, involving genome instability and mutation, epigenetic modification, chronic inflammation, immune evasion, metabolic regulation, and activation of invasion and metastasis. The review also discusses the potential of intratumoural microbiota in cancer prognosis and therapy, as well as the challenges in this field. The findings suggest that intratumoural microbiota may directly or indirectly regulate host epigenetic modifications, including DNA modification, histone modification, RNA modification, and non-coding RNA. However, the molecular mechanisms underlying these changes need further investigation. Chronic inflammation is closely associated with most cancer types, and intratumoural microbiota can activate inflammatory signalling pathways, promoting tumour progression. Immune evasion is another key mechanism, with intratumoural microbiota promoting an immunosuppressive microenvironment and immune cell inactivation. Metabolic regulation is also a critical aspect, with microbiota altering human metabolism and contributing to various metabolic diseases and cancers. The review highlights the potential of intratumoural microbiota as a therapeutic target for supporting immunotherapy, with their effects being context-dependent and requiring further clarification.