A dinitrophenol-based colorimetric chemosensor sequentially sensing Cu²⁺ and S²⁻, HDHT ((E)-2-(2-(2-hydroxy-3,5-dinitrobenzylidene)hydrazineyl)-N,N,N-trimethyl-2-oxoethan-1-aminium), was designed and synthesized. The HDHT selectively detected Cu²⁺ through a color change of yellow to colorless. The calculated detection limit of the HDHT for Cu²⁺ was 6.4 × 10⁻² μM. In the interference test, the HDHT was not considerably inhibited by various metal ions in its detection of Cu²⁺. The chelation ratio of the HDHT to Cu²⁺ was determined as 1:1 by using a Job plot and ESI-MS experiment. In addition, the HDHT–Cu²⁺ complex showed that its color selectively returned to yellow only in the presence of S²⁻. The detection limit of the HDHT–Cu²⁺ complex for S²⁻ was calculated to be 1.2 × 10⁻¹ μM. In the inhibition experiment for S²⁻, the HDHT–Cu²⁺ complex did not significantly interfere with other anions. In the real water-sample test, the detection performance of the HDHT for Cu²⁺ and S²⁻ was successfully examined. The detection features of HDHT for Cu²⁺ and the HDHT–Cu²⁺ for S²⁻ were suggested by the Job plot, UV–Vis, ESI-MS, FT-IR spectroscopy, and DFT calculations.