Abstract Context Sustained hyperglycemia triggers chronic disease, including type 2 diabetes. A considerable volume of research has explored the effects of brown seaweed on plasma glucose control, but equivocal findings have been reported. Objective A systematic review and meta-analysis was conducted to assess the evidence from human randomized controlled trials (RCTs) on the effects of brown seaweed on plasma glucose in healthy, at-risk, and individuals with type 2 diabetes. Data Sources MEDLINE/PubMed, EMBASE, and the Cochrane Library were searched for reports published between 2000 and 2020. Data Extraction Population, intervention, comparator, outcome, and study design data were extracted. Data Analysis Eighteen RCTs met our inclusion criteria. The reported results varied across and between populations. Meta-analyses showed a significant effect, favoring the intervention group for both fasting (mean difference –4.6 [95% CI –7.88, –1.33]) and postprandial (mean difference –7.1 [95% CI –7.4, –6.9]) plasma glucose. Conclusion Brown seaweed and its extracts show potential for preventing and managing hyperglycemia. Our meta-analysis confirms that brown seaweed positively affects plasma glucose homeostasis, with particularly promising postprandial plasma glucose effects. However, further research is needed because no high-quality RCT was identified. Species-specific and dose–response research is also required. Systematic Review Registration PROSPERO registration no. CRD42020187849.