An artificial receptor and a signal transducer have been engineered on a lectin (saccharide-binding protein) surface by a post-photoaffinity labeling modification method. Saccharide binding can be directly and selectively read out by the fluorescence changes of the fluorophore via photoinduced electron transfer (PET) mode. Fluorescence titration with various saccharides reveals that molecular recognition by the artificial receptor is successfully coupled to the native binding site of the lectin, producing a novel fluorescent saccharide biosensor showing modulated specificity and enhanced affinity. Designed co-operativity between artificial and native molecular recognition modules was quantitatively demonstrated by the comparison of the binding affinities, and it represents a new strategy in molecular recognition. By using appropriate artificial receptors and various native lectins, this approach may provide many new semisynthetic biosensors for saccharide derivatives such as glycolipids and glycopeptides/proteins. An extended library of lectin-based biosensors is envisioned to be useful for glycome research, a newly emerging field of the post-genomic era.