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Diabetes mellitus, particularly type 2 diabetes mellitus (T2DM), represents a major global health challenge, driven by the increasing prevalence of obesity and sedentary lifestyles. T2DM is characterized by insulin resistance and progressive β-cell dysfunction, leading to chronic hyperglycemia and multiple complications. Among the molecular targets investigated for therapeutic intervention, protein tyrosine phosphatase 1B (PTP1B) has emerged as a key negative regulator of insulin signaling. By dephosphorylating the insulin receptor and its downstream substrates, PTP1B attenuates insulin action and contributes to metabolic dysfunction. In addition to its role in glucose homeostasis, PTP1B is implicated in obesity, diabetic complications, neurodegenerative disorders, and cancer, highlighting its relevance as a multifunctional therapeutic target. However, the development of PTP1B inhibitors remains challenging due to the highly conserved and polar nature of its catalytic site, which limits selectivity and cell permeability. Recent research has focused on alternative strategies, including allosteric modulation and multi-site inhibition, to overcome these limitations. This review provides a comprehensive overview of PTP1B inhibitors from both synthetic (2019-2025) and natural sources, with particular emphasis on natural products reported from 2022 onwards, while including selected earlier studies to provide historical context and illustrate representative structural classes and inhibition mechanisms. Although PTP1B remains an attractive therapeutic target, its clinical validation for diabetes treatment has yet to be achieved. Continued advances in medicinal chemistry and allosteric modulation may help overcome the current translational barriers.