Aims To investigate the effects of high‐polyphenol chocolate upon endothelial function and oxidative stress in Type 2 diabetes mellitus during acute transient hyperglycaemia induced following a 75‐g oral glucose challenge. Methods Ten subjects with Type 2 diabetes underwent a double‐blinded randomized controlled crossover study. A 75‐g oral glucose load was used to induce hyperglycaemia, which was administered to participants 60 min after they had ingested either low (control) or high‐polyphenol chocolate. Participants undertook testing at weekly intervals, following an initial cocoa‐free period. Endothelial function was assessed by both functional [reactive hyperaemia peripheral artery tonometry (EndoPAT‐2000) and serum markers (including intercellular adhesion molecule 1, P‐selectin and P‐selectin glycoprotein ligand 1]. Urinary 15‐F2t‐isoprostane adjusted for creatinine was used as an oxidative stress marker. Measurements were made at baseline and 2 h post‐ingestion of the glucose load. Results Prior consumption of high‐polyphenol chocolate before a glucose load improved endothelial function (1.7 ± 0.1 vs. 2.3 ± 0.1%, P = 0.01), whereas prior consumption of control chocolate resulted in a significant increase in intercellular adhesion molecule 1 (321.1 ± 7.6 vs. 373.6 ± 10.5 ng/ml, P = 0.04) and 15‐F2t‐isoprostane (116.8 ± 5.7 vs. 207.1 ± 5.7 mg/mol, P = 0.02). Analysis of percentage changes from baseline comparing control and high‐polyphenol chocolate showed a significant improvement for high‐polyphenol chocolate in both measures of endothelial function (P < 0.05) and for urinary 15‐F2t‐isoprostane (P = 0.04). Conclusion High‐polyphenol chocolate protected against acute hyperglycaemia‐induced endothelial dysfunction and oxidative stress in individuals with Type 2 diabetes mellitus.