OBJECTIVE Signal transducer and activator of transcription 3 (STAT3) has been implicated in the development and progression of various solid tumors. We examined the efficacy of STAT3 inhibition as a novel therapeutic option for head and neck squamous cell carcinoma (HNSCC). METHODS Activation and expression of STAT3 and hypoxia-inducible factor-1α (HIF-1α) in HNSCC cell lines were assessed by immunoblots. The small molecule inhibitor, Stattic, was used to target STAT3 in HNSCC cell lines. MTT assays were performed to determine the effect of STAT3 inhibition on HNSCC cell viability, while clonogenic survival assays were used to assess the ability of Stattic to sensitize HNSCC cells to radiation therapy. We also examined the effect of Stattic on tumor growth and radiosensitivity in vivo using an orthotopic xenograft model of HNSCC. RESULTS Stattic effectively inhibited STAT3 activation and expression, resulting in decreased cell survival and proliferation and increased radiosensitivity. STAT3-mediated HIF-1α expression was also reduced in response to Stattic treatment. Oral administration of Stattic significantly reduced the growth of HNSCC tumors in a murine orthotopic xenograft, and analysis of tumor lysates confirmed decreased STAT3 phosphorylation. CONCLUSIONS STAT3 inhibition modulates HIF-1α expression, resulting in decreased tumor growth and possible enhanced radiosensitivity in HNSCC. Our results provide support for further exploration of STAT3 as a novel molecular therapeutic target in HNSCC.