RASSF1A represents one of the eight isoforms of the RASSF1 gene. RASSF1A is a tumor suppressor gene whose inactivation influences tumor initiation and development. In cancer, RASSF1A is frequently inactivated by mutations, loss of heterozygosity and, most commonly, by promoter hypermethylation. As epigenetic inactivation of RASSF1A was detected in various cancer types, it was extensively investigated and nowadays, the research on RASSF1A promoter methylation proceeds in the light of an epigenetic tumor biomarker. Analyses of DNA methylation of genes involved in carcinogenesis such as RASSF1A are currently done mostly on genomic DNA (gDNA). Simultaneously, cell-free DNA (cfDNA) from liquid biopsies has lately been developed as an early cancer diagnostic tool. This review discusses the evidence on aberrantly methylated RASSF1A in gDNA and cfDNA from different cancer types and its utility for early cancer diagnosis, prognosis, and surveillance. Furthermore, methylation frequencies of RASSF1A in gDNA and cfDNA were compared in various cancer types. The weaknesses and strengths of the investigations mentioned above are discussed. In conclusion, although the importance of RASSSF1A methylation in relation to cancer was established, and it became included in several diagnostic panels, the evidence of its diagnostic utility is still experimental and not yet implemented in standard clinical health care.