Background: Low-glycemic index (GI) diets are thought to reduce postprandial glycemia, resulting in more stable blood glucose concentrations. Objecitve: We hypothesized that low-GI diets would be superior to other diet types in lowering measures of blood glucose control in people with type 1 or type 2 diabetes, or impaired glucose tolerance. Methods: We searched PubMed, the Cochrane Library, EMBASE, and clinical trials registries for published and unpublished studies up until 1 March, 2019. We included 54 randomized controlled trials in adults or children with impaired glucose tolerance, type 1 diabetes, or type 2 diabetes. Continuous data were synthesized using a random effects, inverse variance model, and presented as standardized mean differences with 95% CIs. Results: Low-GI diets were effective at reducing glycated hemoglobin (HbA1c), fasting glucose, BMI, total cholesterol, and LDL, but had no effect on fasting insulin, HOMA-IR, HDL, triglycerides, or insulin requirements. The reduction in fasting glucose and HbA1c was inversely correlated with body weight. The greatest reduction in fasting blood glucose was seen in the studies of the longest duration. Conclusions: Low-GI diets may be useful for glycemic control and may reduce body weight in people with prediabetes or diabetes.