Viral diseases continue to pose a major and enduring threat to public health worldwide, as evidenced by recurring seasonal outbreaks and widespread pandemics that result in substantial illness and death. Although conventional antiviral agents offer therapeutic value, their overall efficacy is often hampered by the emergence of resistant strains, potential toxicity, high treatment costs, and a limited spectrum of activity, particularly in addressing newly emerging or mutating viruses. These shortcomings have spurred increasing interest in plant-derived phytochemicals as alternative antiviral agents. In this context, this chapter explores the antiviral potential of phytoconstituent classes such as alkaloids, flavonoids, coumarins, lignans, terpenes, and terpenoids, which have demonstrated the ability to inhibit viral replication, target viral enzymes, or disrupt host–virus interactions in infections caused by HIV, HSV, hepatitis viruses, dengue, influenza, and SARS-CoV-2. Additionally, nanotechnology-based delivery systems such as dendrimers, polymeric nanoparticles, liposomes, nanoemulsions, and so on, have been recognized for their potential to enhance the therapeutic performance of phytochemicals by addressing the drawbacks of free phytochemicals.