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Background: Diabetes mellitus is a chronic metabolic disorder of global concern, and the adverse effects associated with existing synthetic hypoglycaemic agents have generated sustained interest in plant-derived alternatives. Terminalia arjuna (Roxb.) Wight & Arn. (Combretaceae), a bark long employed in Ayurveda as a cardiotonic and astringent tonic, has also been credited with antidiabetic and antioxidant properties. Objective: The present study was designed to carry out pharmacognostical and phytochemical standardization of T. arjuna bark and to evaluate the in vitro dipeptidyl peptidase-4 (DPP-4) inhibitory potential of its ethanolic extract. Methods: Authenticated bark was subjected to macroscopic and microscopic (powder) evaluation, physicochemical analysis (ash values), Soxhlet extraction with ethanol, preliminary qualitative phytochemical screening, fluorescence analysis and thin-layer chromatography (TLC). DPP-4 inhibitory activity of the extract (50–800 μg/mL) was assessed by a fluorimetric enzyme assay using sitagliptin as the reference standard. Results: Powder microscopy revealed rosette and prismatic calcium-oxalate crystals, stone cells, lignified fibres and xylem elements characteristic of Terminalia bark. Total ash, acid-insoluble ash and water-soluble ash were 11.30 ± 0.25, 1.41 ± 0.39 and 18.14 ± 0.03% w/w, respectively. Ethanolic extraction gave a dark-brown semi-solid extract in 8.25% w/w yield. Qualitative screening indicated glycosides, triterpenoids, flavonoids, saponins, alkaloids, phytosterols, phenolics and tannins; TLC resolved five bands (Rf 0.12–0.88). The extract inhibited DPP-4 in a concentration-dependent manner (9–65% over 50–800 μg/mL) with an IC50 of 545.8 μg/mL, compared with 224 μg/mL for sitagliptin. Conclusion: The ethanolic extract of T. arjuna bark exhibits measurable, dose-dependent DPP-4 inhibitory activity, providing preliminary in vitro support for its traditional antidiabetic use. Bioassay-guided isolation and in vivo validation are warranted before therapeutic extrapolation.