Although several studies have evaluated the effect of folate supplementation on diabetes biomarkers among patients with metabolic diseases, findings are inconsistent. This review of randomized controlled trials (RCTs) was performed to summarize the evidence on the effects of folate supplementation on diabetes biomarkers among patients with metabolic diseases. Randomized-controlled trials (RCTs) published in PubMed, EMBASE, Web of Science and Cochrane Library databases up to 1 September 2017 were searched. Two review authors independently assessed study eligibility, extracted data, and evaluated risk of bias of included studies. Heterogeneity was measured with a Q-test and with I2 statistics. Data were pooled by using the fix or random-effect model based on the heterogeneity test results and expressed as standardized mean difference (SMD) with 95% confidence interval (CI). A total of sixteen randomized controlled trials involving 763 participants were included in the final analysis. The current meta-analysis showed folate supplementation among patients with metabolic diseases significantly decreased insulin (SMD -1.28; 95% CI, -1.99, -0.56) and homeostasis model assessment of insulin resistance (HOMA-IR) (SMD -1.28; 95% CI, -1.99, -0.56). However, folate supplementation did not affect fasting plasma glucose (FPG) (SMD -0.30; 95% CI, -0.63, 0.02) and hemoglobin A1C (HbA1c) (SMD -0.29; 95% CI, -0.61, 0.03). The results of this meta-analysis study demonstrated that folate supplementation may result in significant decreases in insulin levels and HOMA-IR score, but does not affect FPG and HbA1c levels among patients with metabolic diseases.