Type 2 diabetes mellitus (DM2) is characterised by high levels of blood glucose due to relative insulin deficiency caused through pancreatic beta-cell dysfunction and insulin resistance. The use of traditional plants as complementary therapies has been growing due to their effect on health, including antioxidant and anti-inflammatory activity as well as their effect against diabetes mellitus and cardiovascular disease. The aims of this study were to investigate (1) the effect of aqueous cinnamon extract (6 g cinnamon burmannii/100 mL) on postprandial glycaemia levels in adults with DM2 and (2) the total phenols content and antioxidant evaluation. This study is a single-blind randomised controlled clinical trial. Participants were randomly assigned to intervention (n = 18) or control group (n = 18). The control group was given only a glucose solution for an oral glucose tolerance test (OGTT) and the intervention group was given a glucose solution for an OGTT immediately followed by a cinnamon aqueous extract. Results show that the ingestion of aqueous cinnamon extract (6 g) has no significant effect on postprandial glycaemia over time in patients with DM2 compared with the control group. However, the phenol content analysis showed that cinnamon extract possesses a considerable antioxidant activity and inhibition capacity of reactive oxygen species. Authors conclude by recommending the intake of aqueous cinnamon extract as a source of natural antioxidants due to its high content in these compounds and respective antioxidant activity.