Background: MicroRNAs (miRs) have been shown to play an important role in tumorigenesis, including in head and neck squamous cell carcinoma (HNSCC). The miR-34a family is thought to play a role in tumor suppression, but the exact mechanism of action in HNSCC is not well understood. In addition, miR-34a is generally down-regulated HNSCC, but the role of chromosomal changes and mutation status on miR-34a expression remain unknown. Methods: Differential expression of miR-34a-3p, MET, and genomic alterations were assessed in the Cancer Genome Atlas (TCGA) datasets as well as primary HNSCC and adjacent normal tissue. The biological functions of miR-34a in HNSCC were investigated in samples derived from primary human tumors. The expression of MET was evaluated using immunohistochemistry, and the molecular interaction of miR-34a and MET were demonstrated by RNA pulldown, RNA immunoprecipitation, and rescue experiments. Lastly, mouse xenograft and locked nucleic acid anti-miRs were used to evaluate the clinical relevance of miR-34a in HNSCC tumor growth and modulation of the tumor microenvironment in vivo.Results: Chromosome arm 1p loss and P53 mutations are both associated with lower levels of miR-34a. In HNSCC, miR-34a acts as a tumor suppressor and physically interacts with and functionally targets the proto-oncogene MET. We found that miR-34a suppresses HNSCC carcinogenesis, at least in part, by downregulating MET, consequently inhibiting HNSCC proliferation. Moreover, ectopic expression of miR-34a reduces HNSCC cell proliferation and tumor burden in vitro and in vivo, represses expression of genes involved in epithelial-mesenchymal transition, and negates the oncogenic effect of MET in mouse tumors. In HNSCC patient samples, higher levels of miR-34a are significantly associated with a higher frequency of Th1 cells, myeloid cells, and regulatory T cells. Consistent with these findings, inhibition of miR-34a in an in vivo model of HNSCC leads to an increased number of immunosuppressive PDL1-expressing tumor-associated macrophages in the tumor microenvironment. Conclusions: Our results demonstrate that miR-34a directly targets MET and maintains anti-tumor immune activity, and could potentially represent a new therapeutic approach for HNSCC.