Head and neck squamous cell carcinomas (HNSCCs) develop from the mucosa of the oral cavity, pharynx, and larynx. The methylation levels of Septin 9 (SEPT9) and short stature homeobox 2 (SHOX2) genes in ccfDNA are considered pan-cancer biomarkers and have shown prognostic value in preliminary reports in HNSCC. Liquid biopsy is a non-invasive procedure that collects tumor-derived molecules including circulating cell-free DNA (ccfDNA). Here, we developed a ddPCR-based assay to detect the DNA methylation levels of plasma circulating SEPT9 and SHOX2 in patients with HNSCC. We first set up the assay on commercial methylated and un-methylated DNA. Then, the dynamic changes of the methylation levels of SEPT9 and SHOX2 were quantified in 20 patients during the follow-up. The results highlighted: i) the capability of the ddPCR-based assay in detecting very low copies of methylated molecules; ii) the significant decrease in methylation levels of SEPT9 and SHOX2 in plasma of HNSCC patients at the first time points of follow-up respect to T0; iii) a different trend of longitudinally DNA methylation variations associable to the clinicopathological features of the patients. The absolute quantification of the methylation levels of SEPT9 and SHOX2 in HNSCC may be used for risk stratification and disease monitoring.