Viral infections constitute one of the foremost challenges to global public health, imposing substantial morbidity, mortality, and socioeconomic burden across diverse geographic settings. Despite considerable progress in antiviral pharmacotherapy, current conventional strategies—including nucleoside analogues, protease inhibitors, and neuraminidase inhibitors—are undermined by rapidly emerging drug resistance, restricted therapeutic spectra, and adverse side-effect profiles that limit long-term applicability. Against this backdrop, plant-based therapies have attracted intensified scientific scrutiny as potential sources of novel antiviral agents endowed with multi-target pharmacological capabilities. This review critically examines the antiviral properties of phytochemicals, with particular focus on the major bioactive classes encompassing flavonoids, alkaloids, terpenoids, and polyphenols. The mechanisms by which these compounds exert antiviral activity are evaluated in depth, encompassing inhibition of viral attachment and cellular entry, suppression of replication machinery, interference with protein synthesis and viral assembly, and immunomodulatory modulation of host defence responses. Evidence derived from in vitro experimental systems, animal models, and early-phase clinical investigations is synthesised and critically appraised. Special attention is directed to translational challenges including pharmacokinetic limitations, bioavailability constraints, variability in phytochemical composition, and regulatory hurdles that impede clinical advancement. Contemporary formulation strategies, including nanoparticulate delivery platforms and standardised extract technologies, are discussed in the context of improving therapeutic efficacy. This review concludes by identifying priority areas for future investigation and underscoring the considerable translational promise of plant-derived antiviral compounds.