A highly stable copper nanoclusters (CuNC) carrying 4-chlorothiophenol as a protective ligand is described. They display self-assembly-induced emission with excitation/emission maxima at 330/605 nm even in neutral or alkaline aqueous environment. The fluorescence of these CuNC is quenched by Hg(II). Quenching is mainly ascribed to the formation of a complex formed via Hg-S bonding between the Hg(II) ions and the ligand. This destroys the ordered architectures of the assembled CuNC. The assay enables Hg(II) to be determined with good sensitivity and a linear response ranging from 1 to 500 nM Hg(II) with a 0.3 nM limit of detection. In addition, the method was implemented in a test strip (which undergoes a color change from red to blue) that can be used for visual determination of Hg(II) in complex environmental water samples. Graphical abstractNovel and highly selective fluorimetric and colorimetric methods have been designed for mercury(II) ions determination based on stable self-assembly-induced emission of copper nanoclusters.