Cancer still poses a significant threat to global health, with early diagnosis playing a critical role in improving treatment outcomes. The current diagnostic approaches are often invasive, expensive, and unsuitable for large-scale screening. Against this background, salivary diagnostics have emerged as a novel non-invasive diagnostic tool. The biofluid saliva, which can be easily collected, reflects the presence of a wide range of biomolecules, including DNA, RNA, proteins, metabolites, and microbiota, which accurately represent the body’s health status. The current literature was searched through the use of keywords including ‘saliva,’ ‘salivary biomarkers,’ ‘tumor markers,’ and ‘cancer,’ using the scientific databases PubMed, Embase, and Scopus, through November 2024. Recent breakthroughs in the field of salivaomics have discovered many salivary markers with the potential to be used in the diagnosis of different cancers, including prostate, breast, pancreatic, lung, and oral cancers. This narrative review provides an original, cross-tumor synthesis of the diagnostic applications of salivary biomarkers in five major malignancies. By integrating findings across heterogeneous sources, the paper aims to outline a conceptual framework that may guide future research and clinical translation.