We have demonstrated a time-resolved luminescence (TRL) assay for the sensitive and selective detection of thyroglobulin (Tg), a thyroid cancer marker, in homogeneous solution using water-soluble alpha-D-mannose-conjugated Au nanodots (Man-Au NDs). The Man-Au NDs (1.7 nm in diameter) were prepared from the reaction between Au nanoparticles (2.9 nm in diameter) with 11-mercapto-3,6,9-trioxaundecyl-alpha-D-mannopyranoside (Man-RSH; 0-10 mM). We have found that the luminescence intensity and lifetimes of Man-Au NDs separately become stronger and longer, respectively, upon increasing the concentration of Man-RSH. The Man-Au NDs possess several interesting optical properties, including long luminescence lifetime (> 500 ns) and large Stokes' shift (> 150 nm). The TRL assay is based on the competition between Tg and Man-Au NDs for interacting with concanavalin A (Con A). The fluorescence intensities integrated over the region of 50-400 ns increase upon increasing the concentration of Tg over the range 0.1-2.5 nM. The Con A/Man-Au NDs system in a label-free manner allows the detection of Tg at concentrations as low as 90 pM in the presence of BSA at 50 microM by TRL. With its advantages of rapid and specificity, this present approach holds great potential for diagnosis of cancers.