A new type of boronate hydrogel with covalently bound dansyldiethylenetriamine as an indicator has been developed; the gel networks are based on boronate esterification of poly(vinylalcohol) with benzene-1,4-diboronic acid. In this approach, phenylboronic acid-appended N-dansyldiethylenetriamine 1 was newly synthesized to be incorporated into the gel matrix. The resulting gel film showed an absorption band at 336 nm and fluorescence at 511 nm when excited at 340 nm in water. The fluorescence measurements indicated that at neutral conditions using a HEPES buffer, the gel film was selectively quenched after immersion in an aqueous solution of Cu²⁺ for 30 min. The response had minimal interference from other metal ions such as Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Al³⁺, and Pb²⁺, which was noteworthy because dansyldiethylenetriamine alone responds to Hg²⁺ and Ni²⁺ in addition to Cu²⁺ in water. The reversible sensing capability was also evaluated by rinsing the film with an aqueous solution of ethylenediaminetetraacetic acid (EDTA). The gel was found to be a reusable and free-standing film capable of visually detecting Cu²⁺, providing a simple and expedient tool for on-site monitoring of Cu²⁺ in environmental applications such as water analysis.