<p>In this study, DHHN ((E)-N’-(2,5-dihydroxybenzylidene)-3-hydroxy-2-naphthohydrazide), a fast and portable fluorescent chemical sensor for the detection of Zn²⁺ ions, was developed. DHHN exhibited a rapid and selective fluorescence response for Zn²⁺. The limit of detection for Zn²⁺ using DHHN was 1.07 µM, and it was effective for Zn²⁺ detection in the pH range of 7 to 10. DHHN stably detected Zn²⁺ in actual environmental water samples, including tap, drinking, and river water. Importantly, the test strip coated with DHHN could detect Zn²⁺ at a concentration (50 µM) lower than the value (76 µM) specified in the World Health Organization (WHO) guideline. Portably, Zn²⁺ detection using DHHN could be conveniently quantified through a smartphone application. DHHN is the first naphthol-based fluorescent chemosensor to quantify Zn²⁺ using a smartphone. A 1:1 binding mechanism between DHHN and Zn²⁺ was proposed through ESI-MS, Job plot, ¹H NMR titrations, and DFT calculations.</p>