Polyphenols are chemical components found largely in plants. Dark chocolate (DC) is one of the highest sources of polyphenols in foods. Several animal and few human studies have suggested that DC might improve insulin sensitivity and decrease glucose levels. The aim of this study was to determine the effect of polyphenol-rich DC (PRDC) on insulin sensitivity (determined by HOMA and QUICKI) and fasting glucose levels. Sixty-one volunteers with no history of diabetes, hypertension or CVD were enrolled in a randomized controlled parallel trial. Participants randomly received 20 g daily of one of the two different types of DC: placebo DC (with negligible amount of polyphenols) or PRDC (500 mg) for a period of 4 weeks. Anthropometric measurements, blood pressure, blood and saliva samples were collected pre and post intervention. A 3-day diet diary was taken at baseline and at week 3. Data was analysed using a paired Students t-test for within group comparisons and ANCOVA for between group differences (to account for potential baseline imbalances). Results showed a significant lowering effect of PRDC on insulin levels (1.17±1.34 μU/ml, P<0.001), and HOMA-IR (0.2±0.33, P=0.003), and an increasing effect on QUICKI (1.12±0.74, P<0.001), but no significant impact on glucose levels (P=0.159) following the intervention. On the other hand, participants administered placebo showed increases in insulin (0.77±1.56 μU/ml, P=0.014), HOMA-IR (0.27±0.44, P=0.003), and glucose levels (0.44±1.08 mmol/l, P=0.041), with QUICKI levels decreasing (0.35±0.7, P=0.013) after 4 weeks. No changes in blood pressure or salivary glucocorticoid hormones (cortisol and cortisone levels) were noted. These results indicate a beneficial effect of PRDC on improving insulin sensitivity, and possibly on preventing or delaying the onset of diabetes. Results also highlight the potential role of polyphenols in counteracting the negative effects of fat in the diet as previously suggested.