Capsular polysaccharides (CPSs) are high-molecular-mass cell-surface polysaccharides, that act as important virulence factors for many pathogenic bacteria. Several clinically important Gram-negative pathogens share similar systems for CPS biosynthesis and export; examples include Escherichia coli, Campylobacter jejuni, Haemophilus influenzae, Neisseria meningitidis, and Pasteurella multocida. Each CPS contains a serotype-specific repeat-unit structure, but the glycans all possess a lipid moiety at their reducing termini. In E. coli and N. meningitidis, the predominant lipid is a lysophosphatidylglycerol moiety that is attached to the repeat-unit domain of the CPS via multiple residues of 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo), referred to as a poly-Kdo linker. The Kdo residues are β-linked, suggesting that they are synthesized by retaining glycosyltransferases. To date, the only characterized Kdo transferases are the inverting enzymes that catalyze the α-linkages found in lipopolysaccharide. Here, we identify two conserved proteins from CPS assembly systems, KpsC and KpsS, as the β-Kdo-transferases and demonstrate in vitro reconstitution of poly-Kdo linker assembly on a fluorescent phosphatidylglycerol acceptor. KpsS adds the first Kdo residue, and this reaction product is then extended by KpsC. Cross-complementation experiments demonstrate that the E. coli and N. meningitidis protein homologs are functionally conserved.