We have employed mannose-modified gold nanodots (Man–Au NDs) as a luminescence sensor for the detection of the thyroid-cancer marker thyroglobulin (Tg) in homogeneous solutions. The luminescent Man–Au NDs are prepared through the reaction of 2.9 nm-diameter gold nanoparticles (Au NPs) with 11-mercapto-3,6,9-trioxaundecyl-α-D-mannopyranoside (Man-RSH) under the irradiation of a light-emitting diode (LED). We have found that the irradiation enhances the quantum yield (∼11 %), alters the emission wavelength and lifetimes, and shortens the preparation time. A luminescence assay has been developed for Tg based on the competition between Tg and Man–Au NDs for the interaction with the concanavalin A (Con A). Because luminescence quenching of the Man–Au NDs by Con A is inhibited by Tg selectivity, we have obtained a highly sensitive and selective assay for Tg.