The present study evaluated the polyphenolic contents and hypoglycemic, antioxidant, and anti-inflammatory effects of the diethyl ether fraction of Thespesia garckeana using various in vitro and in vivo models. Total phenol and flavonoid contents of the extract were 613.65±2.38 and 152.83±1.56 mg/100 g dry weight, respectively. The extract exhibited in vitro antioxidant activities against DPPH, FRAP, LPO, and ABTS with respective half-maximal inhibitory concentration (IC50) values of 30.91±0.23, 16.81±0.51, 41.29±1.82, and 42.39±2.24 μg/mL. In vitro anti-inflammatory studies using membrane stabilization, protein denaturation, and proteinase activities revealed the effectiveness of the extract with respective IC50 values of 54.45±2.89, 93.62±3.04, and 56.60±2.34 μg/mL, while in vitro hypoglycemic analysis of the extract revealed inhibition of α-amylase (IC5064.59±3.29 μg/mL) and enhancement of glucose uptake by yeast cells. Interestingly, the extract demonstrated in vivo hypoglycemic and anti-inflammatory effects in streptozotocin- (STZ-) induced diabetic and xylene-induced ear swelling models, respectively. In addition, the extract improved insulin secretion, attenuated pancreatic tissue distortion and oxidative stress, and increased the activities of superoxide dismutase (SOD), catalase, and reduced glutathione (GSH), while reducing the concentration of LPO in the diabetic rats. A high-performance liquid chromatography (HPLC) analysis identified the presence of catechin (6.81e-1 ppm), rutin (8.46 e-1 ppm), myricetin, apigenin (4.019 e-1 ppm), and luteolin (15.09 ppm) with respective retention times (RTs) of 13.64, 24.269, 27.781, 29.58, and 32.23 min, and these were subjected to a pharmacoinformatics analysis, which revealed their drug-likeness and good pharmacokinetic properties. A docking analysis hinted at the potential of luteolin, the most abundant compound in the extract, for targeting glucose-metabolizing enzymes. Thus, the present study provides preclinical insights into the bioactive constituents of T. garckeana, its antioxidant and anti-inflammatory effects, and its potential for the treatment of diabetes.