Emerging evidence suggests that microbial dysbiosis plays vital roles in many human cancers. However, knowledge of whether the microbial community in thyroid tumor is related to tumorigenesis remains elusive. In this study, we aimed to explore the microbial community in thyroid tissues and its contribution to papillary thyroid cancer (PTC). In parallel, we performed microbial profiling and transcriptome sequencing in the tumor and adjacent normal tissues of a large cohort of 340 PTC and benign thyroid nodule (BTN) patients. Distinct microbial signatures were identified in PTC, BTN, and their adjacent non-tumor tissues. Intra-thyroid tissue bacteria were verified by means of bacteria staining, fluorescence in situ hybridization, and immunoelectron microscopy. We found that 17 bacterial taxa were differentially abundant in PTC compared with BTN, which included enrichment in PTC of the pathobionts Rhodococcus, Neisseria, Streptococcus, Halomonas, and Devosia, and depletion of the beneficial bacteria Amycolatopsis. These differentially abundant bacteria could differentiate PTC tumor tissues (PTC-T) from BTN tissues (BTN-T) with an area under the curve (AUC) of 81.66%. Microbial network analysis showed increased correlation strengths among the bacterial taxa in PTC-T in comparison with BTN-T. Immune-function-corresponding bacteria (i.e., Erwinia, Bacillus, and Acinetobacter) were found to be enriched in PTC with Hashimoto’s thyroiditis. Moreover, our integrative analysis revealed that the PTC-enriched bacteria had a positive association with key PTC-oncogenic pathway-related genes, including BRAF, KRAS, IRAK4, CTNNB1, PIK3CA, MAP3K7, and EGFR. In conclusion, our results suggest that intratumor bacteria dysbiosis is associated with the thyroid tumorigenesis and oncogenic signaling pathways of PTC.