Dietary fiber has beneficial effects on obesity-related diseases and gut microbiota, contributing a key role in the interaction between dietary metabolism and host metabolism. Our objective was to investigate the cause of the improvement in multiple types of physiological states with seaweedUndaria pinnatifidatreatment on high-fat diet-fed mice and to evaluate whether its consequent anti-adiposity and anti-hyperlipidemic effects are associated with gut microbiota and its metabolomics regulation.U. pinnatifidaadministration in our experiment was shown to significantly decrease high-fat diet-induced body weight gain, as well as epididymal and abdominal adiposity.U. pinnatifidaintake also significantly reduced liver weight and serum triacylglycerol accumulation. We also found that improving effects ofU. pinnatifidaon high-fat diet-induced metabolic dysfunctions were associated with significant increase in specific bacteria, such asBacteroides acidifaciensandBacteroides ovatus, as well as metabolites, including short-chain fatty acids and tricarboxylic acid cycle intermediates. Our result provides a cheap dietary strategy to host metabolism improvement and obesity management.