Chronic diseases, including cardiovascular disorders, diabetes mellitus, cancer, and neurodegenerative conditions, represent a major global health burden characterized by complex multifactorial etiologies involving oxidative stress, inflammation, metabolic dysregulation, and genetic alterations. In recent years, phytochemicals—bioactive compounds derived from plants such as polyphenols, flavonoids, alkaloids, terpenoids, and carotenoids—have gained considerable attention due to their therapeutic potential in the prevention and management of chronic diseases. These natural compounds exert diverse pharmacological effects through modulation of key molecular pathways, including antioxidant defense systems, inflammatory signaling cascades, apoptosis, and cellular metabolism. Phytochemicals such as curcumin, resveratrol, quercetin, and epigallocatechin gallate have demonstrated significant efficacy in targeting disease-specific mechanisms, including inhibition of nuclear factor-kappa B (NF-?B), activation of nuclear factor erythroid 2–related factor 2 (Nrf2), and regulation of mitogen-activated protein kinase (MAPK) pathways. Additionally, emerging evidence highlights the role of phytochemicals in epigenetic regulation and gut microbiota modulation, further enhancing their therapeutic relevance. Despite promising preclinical and clinical findings, challenges such as poor bioavailability, rapid metabolism, and variability in phytochemical content limit their clinical translation. Advances in nanotechnology-based drug delivery systems and formulation strategies are being explored to overcome these limitations and improve therapeutic efficacy. This review provides a comprehensive overview of the mechanistic insights underlying the therapeutic potential of phytochemicals in chronic diseases, emphasizing their molecular targets, pharmacological actions, and future prospects in integrative medicine and drug development.