Although higher dietary intakes of magnesium (Mg) seem to correspond to lower diabetes incidence, research concerning Mg supplementation in people with or at risk of diabetes is limited. Thus, we aimed to investigate the effect of oral Mg supplementation on glucose and insulin-sensitivity parameters in participants with diabetes or at high risk of diabetes compared with placebo. A literature search in PubMed, EMBASE, SCOPUS, Cochrane Central Register of Controlled Trials and Clinicaltrials.gov without language restriction, was undertaken. Eligible studies were randomized controlled trials (RCTs) investigating the effect of oral Mg supplementation vs placebo in patients with diabetes or at high risk of diabetes. Standardized mean differences (SMD) and 95% confidence intervals (CIs) were used for summarizing outcomes with at least two studies; other outcomes were summarized descriptively. Eighteen RCTs (12 in people with diabetes and 6 in people at high risk of diabetes) were included. Compared with placebo (n=334), Mg treatment (n=336) reduced fasting plasma glucose (studies=9; SMD=-0.40; 95% CI: -0.80 to -0.00; I2=77%) in people with diabetes. In conditions in people at high risk of diabetes (Mg: 226; placebo=227 participants), Mg supplementation significantly improved plasma glucose levels after a 2 h oral glucose tolerance test (three studies; SMD=-0.35; 95% CI: -0.62 to -0.07; I2=0%) and demonstrated trend level reductions in HOMA-IR (homeostatic model assessment-insulin resistance; five studies; SMD=-0.57; 95% CI: -1.17 to 0.03; I2=88%). Mg supplementation appears to have a beneficial role and improves glucose parameters in people with diabetes and also improves insulin-sensitivity parameters in those at high risk of diabetes.