Polysaccharides play key roles in biology, including in energy storage, as structural materials, and as modulators of protein interactions and activity. However, polysaccharides are remarkably diverse in molecular configuration, functionalization, linkage types, and degree of branching, and thus, are challenging synthetic targets. To address this fundamental problem, we synthesized enantiopure poly-amido-saccharides (PASs) with defined molecular weights and narrow dispersities using an anionic ring-opening polymerization of a β-lactam sugar monomer. The PASs possess a previously unreported main chain structure composed of pyranose rings linked through the 1- and 2-positions by an amide bond with a-configuration. PASs offer the advantages associated with synthetic polymers, such as greater control over structure and derivitization. At the same time, they preserve many of the structural features of natural polysaccharides, such as a stereochemical regularity and rigid pyranose backbone, that make natural carbohydrate polymers important materials both for their unique properties and useful applications.