<i>Background/Aims:</i> In 2008, the American Diabetes Association recommended low-carbohydrate or low-fat diets for weight management in patients with established type 2 diabetes (T2D), while the amount of monounsaturated fatty acids (MUFA) was not specified. This systematic review focused on the effects of diets high in MUFA versus diets low in MUFA on important risk factors of T2D (i.e. plasma glucose, insulin, homeostasis model assessment of insulin resistance and glycosylated haemoglobin, HbA1c). <i>Methods:</i> Nine randomized controlled intervention trials with a total of 1,547 participants and a running time of at least 6 months, comparing diets high versus low in MUFA among adults with abnormal glucose metabolism (T2D, impaired glucose tolerance and insulin resistant), being overweight or obese, have been included in the meta-analysis. We performed a random effects meta-analysis to determine the weighted mean differences with 95% confidence intervals using the software package Review Manager 5.0.25 of the Cochrane Collaboration. <i>Results:</i> Significant differences in HbA1c were found (weighted mean difference –0.21%, 95% CI –0.40 to –0.02; p = 0.03), favouring the high MUFA groups. In contrast, fasting plasma glucose, fasting plasma insulin as well as the homeostasis model assessment of insulin resistance were not affected by the amounts of MUFA in the dietary protocols. <i>Conclusions:</i> In summary, this systematic review found that high MUFA diets appear to be effective in reducing HbA1c, and therefore, should be recommended in the dietary regimes of T2D.