A colorimetric chemosensor CMF (N′‐[(E)‐(4‐oxo‐4H‐chromen‐3‐yl)methylidene]furan‐2‐carbohydrazide) was applied for detecting Cu2+. CMF showed colorimetric selectivity to Cu2+ through a color variation from colorless to yellow. Binding ratio between CMF and Cu2+ was analyzed by Job plot to be a 2:1. Limit of detection was found to be 0.38 μM. The practicality of CMF was illustrated by the quantification of Cu2+ in real samples. In particular, the CMF‐coated test kit was able to detect the copper ions at a concentration lower than the recommended concentration (31.5 μM) of the World Health Organization (WHO) for copper ion. The binding mode of CMF and copper ions was studied through UV–visible spectroscopy, 1H NMR titration, ESI‐mass analysis, and DFT calculations. Sensing mechanism of CMF to Cu2+ was proposed to be the ligand‐to‐metal charge transfer (LMCT) through DFT calculations.