The hypoglycemic and lipid-lowering effects of glucomannan are widely known, and it is a potential effective treatment for type II diabetes. In this study, we evaluated the effects of glucomannan supplementation on blood-lipid-related indicators, blood-glucose-related indicators, blood pressure (BP), and body weight (BW) in patients suffering from type II diabetes. We searched databases including PubMed, Cochrane, the comprehensive biomedical research database (Embase), Web of Science, and China National Knowledge Infrastructure (CNKI) for literature on glucomannan and type II diabetes. Six randomized controlled trials (RCTs) were eligible (n = 440 participants) to be included in our analysis. Glucomannan not only reduced the total cholesterol (TC) (MD −0.38 [95% CI: −0.61, −0.15], p = 0.001) and low-density lipoprotein (LDL) levels (MD −0.35 [95% CI: −0.52, −0.17], p < 0.0001) compared with the control group, but also reduced the fasting blood glucose (FBG) (MD −1.08 [95% CI: −1.65, −0.50], p = 0.0002), 2 h postprandial blood glucose (P2hBG) (MD −1.92 [95% CI: −3.19, −0.65], p = 0.003), fasting insulin (FINS) (MD −1.59 [95% CI: −2.69, −0.50], p = 0.004), and serum fructosamine (SFRA) levels (SMD −1.19 [95% CI: −1.74, −0.64], p < 0.0001). Our analysis indicates that glucomannan is an effective nutritional intervention for type II diabetes.