Lectin–carbohydrate interactions are the basis of many biological processes and essentially they constitute the language through which intercellular communications are codified. Thus they represent powerful tools in the examination and interpretation of changes that occur on cell surfaces during both physiological and, more importantly, pathological events. The development of optical techniques that exploit the unique properties of luminescent lanthanoid metal complexes in the investigation of lectin–carbohydrate recognition can foster research in the field of ratiometric biosensing and disease detection. Here we report the synthesis of a Tb³⁺-DO3A complex (Tb⊂1) bearing an α-D-mannose residue and the related study of binding affinity with concanavalin A (Con A) labeled with rhodamine-B-isothiocyanate (RITC-Con A). Luminescence spectroscopy and dynamic studies show changes in emission spectra that can be ascribed to a luminescence resonance energy transfer (LRET) from Tb⊂1 (donor) to RITC-Con A (acceptor). The binding constant value between the two species was found to be one order of magnitude larger than those previously reported for similar types of recognition. To the best of our knowledge this is the first example of the use of a pre-organized luminescent lanthanoid complex in the study of carbohydrate–protein interactions by LRET.