Lignan is a large class of plant secondary metabolites, which has long attracted pharmacological interest because of its anti-tumor and estrogenic activities. Forsythia plants are known to produce a wide variety of lignans, such as (-)-matairesinol, (-)-secoisolariciresinol, (+)-pinoresinol, and (+)-phillygenin. The majority of such lignans are accumulated in glucoside forms. However, their glucosylation mechanisms largely remain to be elucidated. Here we describe the sequence, enzymatic activities, and gene expression profiles of UDP-sugar dependent-glycosyltransferases (UGT) from Forsythia koreana through a reverse-genetic approach. A Forsythia UGT, UGT71A18 protein, expressed in E. coli, preferentially glucosylated furofuran-class lignans, including (+)-pinoreisnol, (+)-epipinoreisnol, and (+)-phylligenin. Moroeover, the recombinant UGT71A18 exhibited specificity to UDP-glucose as a glycosyl donor. Gene expression analysis revealed that UGT71A18 is expressed predominantly in leaves and the suspension cell culture of F. koreana, and that the UGT71A18 transcript is upregulated in the transgenic cell culture expressing the RNAi construct of the pinoresinol lariciresinol reductase ( PLR) gene, compared to non-transformants. These results are consistent with the remarkable elevation of pinoresinol glucosides in the PLR-RNAi lines. Collectively, the present data strongly suggests that UGT71A18, in part, is responsible for glucosylation of furofuran-class lignans, including (+)-pinoresinol and/or structurally related lignans in vivo.