Lipopolysaccharide O-antigen is an attractive candidate for immunotherapeutic strategies targeting antibiotic-resistant Klebsiella pneumoniae. Several K. pneumoniae O-serotypes are based on a shared O2a-antigen backbone repeating unit: (→ 3)-α-Galp-(1 → 3)-β-Galf-(1 →). O2a antigen is synthesized on undecaprenol diphosphate in a pathway involving the O2a polymerase, WbbM, before its export by an ATP-binding cassette transporter. This dual domain polymerase possesses a C-terminal galactopyranosyltransferase resembling known GT8 family enzymes, and an N-terminal DUF4422 domain identified here as a galactofuranosyltransferase defining a previously unrecognized family (GT111). Functional assignment of DUF4422 explains how galactofuranose is incorporated into various polysaccharides of importance in vaccine production and the food industry. In the 2.1-Å resolution structure, three WbbM protomers associate to form a flattened triangular prism connected to a central stalk that orients the active sites toward the membrane. The biochemical, structural and topological properties of WbbM offer broader insight into the mechanisms of assembly of bacterial cell-surface glycans.