Enzyme inhibition constitutes one of the most pivotal strategies in modern drug development, enabling targeted modulation of pathological biochemical pathways across diverse disease states. Despite the widespread clinical utility of synthetic enzyme inhibitors, their use is increasingly complicated by concerns over systemic toxicity, suboptimal pharmacokinetic profiles, and the emergence of drug resistance. These limitations have reinvigorated scientific interest in phytochemicals, a structurally diverse class of plant-derived secondary metabolites with demonstrated enzyme-inhibitory activity. This review examines the role of phytochemicals, including flavonoids, alkaloids, terpenoids, and polyphenols, as mechanistically diverse enzyme inhibitors in the context of contemporary drug discovery. We analyse the fundamental principles of competitive, non-competitive, uncompetitive, allosteric, and covalent inhibition, and delineate how each modality is exploited by specific phytochemical scaffolds. Molecular interactions at enzyme active sites, including hydrogen bonding, hydrophobic contacts, pi-pi stacking, and van der Waals forces, are discussed in relation to structure-activity relationships that govern inhibitory potency and selectivity. Key therapeutic applications encompassing metabolic disorders such as type 2 diabetes, oncological targets, and neurodegenerative pathways are reviewed in depth. Additionally, computational approaches including molecular docking, pharmacophore modelling, and machine learning are evaluated for their role in accelerating phytochemical-based lead identification. Challenges in translating preclinical findings to clinical efficacy, including bioavailability limitations, regulatory hurdles, and standardization issues, are critically discussed. Future directions in the field encompass nanotechnology-mediated delivery, combination therapy strategies, and integration of artificial intelligence in phytochemical drug design.