In this paper, we present a new class of carbonic anhydrase (CA) inhibitor that was designed to selectively target the extracellular domains of the cancer-relevant CA isozymes. The aromatic moiety of the classical zinc binding sulfonamide CA inhibitors is absent from these compounds and instead they incorporate a hydrophilic mono- or disaccharide fragment directly attached to the sulfonamide group to give S-glycosyl primary sulfonamides (1-10). The inhibition properties of these compounds at the physiologically abundant human CA isozymes I and II and cancer-associated IX and XII were determined, and all compounds had moderate potency with K(i)s in the micromolar range. We present the crystal structures of anomeric sulfonamides 4, 7, and 10 and the sugar sulfamate drug topiramate in complex with human recombinant CA II. From these structures, we have obtained valuable insights into ligand-protein interactions of these novel carbohydrate-based sulfonamides with CA.