A multi-functional fluorescent probe based on PzDPM (10-ethyl-3,7-di(pyrimidin-4-yl)-10H-phenothiazine) for Hg²⁺, Cu²⁺ and ClO⁻ has been synthesized and characterized. The probe comprises an electron-donating fluorophore core of 10-ethylphenothiazine and two Hg²⁺-specific chelating arms of pyrimidin-4-yl. The 10-ethylphenothiazine also acts as a Cu²⁺/ClO⁻-specific reactive moiety. PzDPM exhibits green fluorescence and selectively senses Hg²⁺/Cu²⁺ upon coordination/reaction in acetonitrile (MeCN), and behaves as a turn-off chemosensor or ratiometric chemodosimeter, respectively. On the other hand, PzDPM is very weakly emissive in aqueous solution but acts as an excellent turn-on chemodosimeter for ClO⁻ in 1 : 4 (v/v) MeCN : Tris–HCl (10 mM, pH = 7.0) with a maximum fluorescent intensity increase of over 110-fold. The probe PzDPM allows the determination of Hg²⁺, Cu²⁺ and ClO⁻ at 10⁻⁷ M levels with satisfactory selectivity.