Immune checkpoint inhibitors (ICIs) are effective in treating recurrent/metastatic head and neck squamous cell carcinoma (HNSCC), but only 20% of patients achieve durable responses. This study evaluated circulating tumor DNA (ctDNA) as a real-time biomarker for monitoring treatment response in HNSCC. The SHIZUKU-HN study prospectively collected and analyzed serial plasma samples (n = 27) from HNSCC patients undergoing ICIs, using Guardant360 to assess ctDNA variant allele frequency (VAF) and genetic mutations. Tumor volumes were quantified using 3D reconstruction of CT scans, and data from Japan's C-CAT database (n = 2255) provided insights into ctDNA testing in HNSCC. C-CAT data showed that ctDNA testing was underutilized, performed in only 7% of head and neck cancer cases. In SHIZUKU-HN, mean VAF significantly correlated with tumor volume (Spearman's ρ = 0.70, = 0.001), often preceding radiographic progression. BRAF and APC mutations disappeared in partial responders, while GNAS mutations varied. EGFR and PIK3CA amplifications, detectable via ctDNA but missed in tissue biopsies, indicated emerging resistance mechanisms. The SHIZUKU-HN study demonstrates the potential of ctDNA as a dynamic biomarker in HNSCC, offering early insights into treatment efficacy and informing personalized ICI therapy.