Commelina communis L., a functional food and herbal plant in Asia, has been used against obesity, diabetes, and infections for centuries. A growing body of studies has demonstrated that indigestible polysaccharides are significant in obesity management. However, the structures and bioactivities of homogeneous polysaccharides from C. communis remain unclear. This study presented the structural characterization, simulated digestion, and human gut Bacteroides proliferation promotion activity of a novel homogeneous polysaccharide (CCB-3) from C. communis. The results showed that CCB-3 was an arabinoglucuronoxylan, primarily composed of arabinose, galactose, xylose, glucuronic acid (GlcA), and 4-O-methyl GlcA with a molecular weight (Mw) of 58.8 kDa. Following a 6-hour exposure to simulated gastrointestinal fluid, the Mw of CCB-3 remained unchanged, revealing that CCB-3 was an indigestible polysaccharide. Notably, CCB-3 could promote the proliferation of B. thetaiotaomicron, B. ovatus, and B. cellulosilyticus and produce short-chain fatty acids (SCFAs) and 1,2-propanediol. These findings might shed light on the discovery of polysaccharide-based leading compounds from C. communis against obesity.