Introduction: Leptomeningeal disease (LMD) in diffuse midline gliomas can lead to devastating symptoms such as severe pain, urinary incontinence, and tetraparesis, with limited treatment options. In the present study, we determined whether the detection of H3F3A K27M-mutant droplets in cerebrospinal fluid (CSF) circulating tumor DNA (ctDNA) could serve as a reliable biomarker for the detection of LMD in diffuse midline gliomas. Methods: Twenty-four CSF samples were obtained from 21 diffuse midline glioma patients. Histological confirmation of H3F3A K27M mutation was obtained in 9 (42.8%) cases. ctDNA was extracted from CSF, and H3F3A K27M-mutant and wildtype droplets were detected using digital droplet PCR (ddPCR). LMD was diagnosed by CSF cytology and/or pre- and post-contrast head and whole spine MR imaging. Results: The number of H3F3A K27M-mutant droplets (median 7 [range 0-297] vs median 0 [range 0-2]; p <.0001) and variant allele frequency (VAF) (median 50.0% [range 7.5-87.5%] vs median 0.0% [range 0.0-40.0%]; p <.0001)) were significantly higher in the LMD/early LMD group compared to no LMD group. In two cases (Case 4 and Case 11) without clinical evidence of LMD, multiple H3F3A K27M-mutant droplets were detected in the CSF ctDNA. In those cases, extensive spinal dissemination was detected 175 and 176 days after the initial liquid biopsy. One case (Case 15) with high Schlafen11 (SLFN11) expression in the tumor responded well to treatment for LMD and survived for 532 days after the diagnosis of LMD. Conclusion: This study provides evidence that detecting H3F3A K27M-mutant droplets in CSF ctDNA is diagnostic for LMD and is more sensitive than traditional methods such as CSF cytology and MR imaging.