Liquid biopsy is a test that allows for the diagnosis and analysis of cancer by sampling cancer cells or byproducts present in biological fluids such as blood or urine. It has the potential to create a new paradigm in oncologic care, being a less invasive approach than conventional tissue biopsy. Liquid biopsy has multifaceted applications for longitudinal disease monitoring in terms of surveillance, treatment response, and identification of emerging resistance mechanisms. Multiple assays currently exist or are in development for detecting circulating tumor cells, DNA, RNA, exosomes, proteins, fragmentomic markers, and metabolomes. Here, we review the applications of liquid biopsy in gastrointestinal cancers, emphasizing its use in both perioperative and advanced settings. We also examine its role in screening, diagnostics, and other cancer-related scenarios.