A naphthylthiourea-modified cyclodextrin (1) and its urea derivative (2) were synthesized, and their fluorescence behaviors in the presence of various metal ions were investigated. Significantly, 1 showed a highly sensitive and selective fluorescence sensing ability for Hg²⁺ over other metal ions in both water and living cells. That is, the addition of Hg²⁺ to an aqueous solution of 1 gave a significantly enhanced fluorescence at ∼380 nm. In contrast, the addition of other metal ions induced negligible fluorescence changes. The possible mechanism may be due to the transformation of thiourea to urea by Hg²⁺-induced desulfurization in water.