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Background Intratumoral bacteria influence the progression and treatment response of solid tumors through multiple mechanisms. Oral squamous cell carcinoma (OSCC) is a common malignant tumor in the head and neck; however, the role of intratumoral bacteria in OSCC initiation and progression remains poorly understood.Methods We integrated 21 public 16S rRNA gene amplicon sequencing (16S rRNA-seq) datasets (comprising 954 normal and 1,627 OSCC samples) to profile oral microbiota dysbiosis across 4 sample types (saliva, oral rinse, swab, and tissue). Subsequent analysis via five-region 16S rRNA-seq and fluorescence in situ hybridization revealed a specific species enriched in OSCC tissues. The functional role of this bacterium and its underlying mechanism were then elucidated using in vitro and in vivo models, including germ-free mice.Results Our analysis revealed a reduced diversity of the oral microbiota in patients with OSCC, along with a significant enrichment of the Capnocytophaga in swab and tissue samples. Capnocytophaga leadbetteri (C. leadbetteri), a species within Capnocytophaga, was further confirmed to be specifically enriched in OSCC tissues. Functional studies demonstrated that C. leadbetteri alone is sufficient to induce epithelial hyperproliferation and a protumorigenic inflammatory niche, and it promotes established OSCC progression. Mechanistically, C. leadbetteri activates the TLR4/MyD88/NF-κB pathway in OSCC cells, stimulating tumor cell proliferation and the expression of chemokines (Cxcl1, Cxcl2, Ccl5, and Ccl7). This leads to the recruitment of tumor-associated neutrophils and establishes a protumorigenic microenvironment.Conclusions Our findings establish a protumorigenic role for intratumoral C. leadbetteri in OSCC and highlight its potential as a novel diagnostic and therapeutic target.