Type 2 diabetes mellitus (T2DM) imposes a growing global health burden, and tea-processing by-products (e.g., stems) remain underutilized despite being rich in bioactive polysaccharides. This study characterized tea stem polysaccharides (TSPs) and assessed their hypoglycemic effects and the underlying mechanisms in T2DM mice. TSP was identified as an acidic heteropolysaccharide rich in galacturonic acid. Four-week oral TSP administration significantly reduced fasting blood glucose (FBG, p < 0.01) and improved glucose tolerance, as evidenced by a decreased area under the curve of the oral glucose tolerance test (OGTT-AUC) (p < 0.01), while dose-dependently alleviating pancreatic, hepatic and cecal histopathological damage. Integrated 16S rRNA sequencing and untargeted metabolomics revealed that TSP selectively enriched Lactobacillus and Ligilactobacillus and modulated amino acid and carbon metabolism pathways. Targeted metabolomics further showed that TSP induced an upward trend in the levels of some branched short-chain fatty acids (SCFAs). As a prebiotic, TSP improves T2DM metabolic disorders via the gut microbiota-metabolite axis, supporting the high-value utilization of tea stem by-products.