Phytochemicals are typically natural bioactive compounds or metabolites produced by plants. Phytochemical-loaded nanocarrier systems, designed to overcome bioavailability limitations and enhance therapeutic effects, have garnered significant attention in recent years. The coronavirus disease 2019 (COVID-19) pandemic has intensified interest in the therapeutic application of phytochemicals to combat viral infections. This review explores nanoparticle-based treatment strategies incorporating phytochemicals for antiviral application, highlighting their demonstrated antiviral mechanisms. It specifically examines the antiviral activities of phytochemical-loaded nanosystems against (i) influenza virus (IAV), respiratory syncytial virus (RSV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2); (ii) mosquito-borne viruses [dengue (DENV), Zika (ZIKV), and Chikungunya (CHIKV)]; and (iii) sexually transmitted/blood borne viruses [e.g. herpes simplex virus (HSV), human papillomavirus (HPV), and human immunodeficiency virus (HIV)]. Furthermore, this review highlights the emerging role of these nanosystems in photodynamic therapy (PDT)-mediated attenuation of viral proliferation, and offers a perspective on the future directions of research in this promising area of multimodal therapeutic approach.