<p><i>Triclisia subcordata</i> leaves’ decoction has traditionally been used in treating diabetes mellitus, but no data from in vivo studies exists. Therefore, the purpose of this research is to justify the use of <i>Triclisia subcordata</i> in treating diabetes. Methanol was used to macerate the pulverized <i>T. subcordata</i> leaves. The antioxidant activities using 2,2-diphenyl-1-picrylhydrazyl, total phenolic (TPC), and flavonoid content (TFC) of the crude extract and hexane, dichloromethane (DCM), ethyl acetate, and aqueous fractions of <i>T. subcordata</i> leaf methanol extract were determined using standard methods. The methanol extract was subjected to GC–MS analysis. Lorke’s method was used to test the toxicity of the crude extract. In alloxan-induced diabetic Wistar rats, the anti-diabetic effect of crude and fractions of <i>T. subcordata</i>, as well as Glibenclamide (standard), was investigated. The aqueous extract of <i>T. subcordata</i> had the lowest IC₅₀ (145.10 ± 3.05 g/mL) for antioxidant activity. The TFC value was 1462 ± 83.97 mgQE/g in the hexane fraction, while the DCM fraction has a TPC value of 106.4 ± 4.343 mgGAE/g. Compounds including 1-aspidospermidin-17-ol, 1-acetyl-19,21-epoxy-15,16-dimethoxy- were detected in the methanol extract. The crude <i>T. subcordata</i> extract was safe even at 5000 mg/kg bw. The in vivo antidiabetic study revealed the percentage reductions in blood glucose levels of dichloromethane fraction-treated rat and n-hexane fraction-treated rats at 150 mg/kg were greater than other fractions and the positive control, glibenclamide, on days 1, 4, and 7 of treatment. This plant’s anti-diabetic activity may be attributed to phenolic compounds and their ability to scavenge free radicals. <i>Triclisia subcordata</i> extract and fractions exhibited antioxidant and antihyperglycemic activities in diabetic rats.</p>