We describe the facile synthesis of stable gold nanoparticle clusters densely functionalized with DNA (DNA-AuNP clusters) using dithiothreitol and monothiol DNA and their thermally reversible assembly properties. The size of the clusters exhibits a very narrow distribution and can be easily controlled by adjusting the stoichiometry of dithiothreitol and DNA, leading to a variety of colors due to the surface plasmon resonance of the AuNP clusters. Importantly, the DNA-AuNP clusters exhibit highly cooperative melting properties with distinctive and diverse color changes depending on their size. The selective and sensitive colorimetric detection of target sequences was demonstrated based upon the unique properties of the DNA-AuNP clusters.