STAT3 is an oncogenic transcription factor that activates cancer cell signaling and induces an immunosuppressive immune environment, making it an attractive therapeutic target. Transcription factors are exceptionally challenging targets and there are no Food and Drug Administration-approved STAT3 inhibitors. We previously reported positive pharmacodynamics of a linear STAT3 decoy oligonucleotide administered intratumorally in a phase 0 trial in patients with head and neck cancer squamous cell carcinoma (HNSCC). Here, we describe the anti-tumor and immune effects of a systemically administered cyclic STAT3 decoy (CS3D) in immunocompetent HNSCC murine models and the safety and efficacy of CS3D in a clinical trial in pet cats with HNSCC. Responders in the clinical trial (35% disease control rate) showed significant differences in selected peripheral blood immune parameters as well as elevated PD-1 expression in the tumors compared with non-responders. These findings support a clinical trial of CS3D in HNSCC patients.