This paper describes the sensing application of water-dispersible dopamine dithiocarbamate decorated gold nanoparticles (DDTC-Au NPs) as sensors for the colorimetric detection of Cu2+ ions in water samples. Dopamine dithiocarbamate (DDTC) molecules were successfully attached on the surfaces of Au NPs and were characterized by using UV-visible, FT-IR. proton NMR, TEM, and DLS (dynamic light scattering). The color of DDTC-Au NPs was changed from purple to blue by the addition of Cu2+ ions at pH 9.0 by using Tris-tricine buffer. These changes were measured by UV-visible spectrometry and DLS. The method was linear in the range of 1-10 mM with correlation coefficient (R-2) 0.999. As a result, the present approach allows the detection of Cu2+ ions at 14.9 x 10(-6) M. DDTC-Au NPs were effectively used as colorimetric sensors for the detection of Cu2+ ions in real samples (tap water).