Abstract A colorimetric probe L based on rhodamine was designed and synthesized. It exhibits very high sensitivity and selectivity towards Cu2+ in an aqueous medium with a color change from colorless to pink. The competitive experiments of probe L showed excellent colorimetric selectivity for Cu2+ over other metal ions. The recognizing behavior of L has been investigated both experimentally and computationally. The mechanism of L for sensing Cu2+ has been demonstrated by IR, ESI-MS, density functional theory calculation and time-dependent density functional theory calculations.