A new rhodamine-derived fluorescent chemodosimeter (1) for Cu²⁺ was synthesized, where a carboxylic acid group was introduced into rhodamine B hydrazide to enhance its water solubility. The probe 1 was weakly fluorescent but displayed a Cu²⁺-selective fluorescence enhancement in aqueous solution. It was found that a 1 : 1 complex was formed between the probe 1 and Cu²⁺ in CH₃CN and rapidly converted into the parent rhodamine molecule upon addition of water. Thus a sensing mechanism of coordination-promoted hydrolysis was proposed based on ESI-MS analysis and DFT calculation. The present probe works in a near pure water solution (only containing 1% CH₃CN as co-solvent), with visible excitation/emission wavelength and a wide pH range of 6–10, making it suitable for fluorescence imaging of Cu²⁺ in the living cells.