The platinum(II) half-lantern dinuclear complexes [{Pt(bzq)(μ-C₇H₄NS₂-κN,S)}₂] (A) and [{Pt(bzq)(μ-C₇H₄NOS-κN,S)}₂] (B) [bzq = benzo[h]quinolinate, C₇H₄NS₂ = 2-mercaptobenzothiazolate, C₇H₄NOS = 2-mercaptobenzoxazolate] in solution of DMSO-H₂O undergo a dramatic color change from yellowish-orange to purple and turn-off phosphorescence in the presence of a small amount of Hg²⁺, being discernible by the naked-eye and by spectroscopic methods. Other metal ions as Ag⁺, Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Ba²⁺, Pb²⁺, Cd²⁺, Zn²⁺ and Tl⁺ were tested and, even in a big excess, showed no interference in the selective detection of Hg²⁺ in water. Job's plot analysis indicated a 1:1 stoichiometry in the complexation mode of Hg²⁺ by A/B. The phosphorescence quenching attributed to the formation of [A/B:Hg²⁺] complexes showed binding constants of K = 1.13 × 10⁵ M^-1 (A) and K = 1.99 × 10⁴ M^-1 (B). The limit of detection has been also evaluated. In addition, dried paper test strips impregnated in DMSO solutions of A and B can detect concentration of Hg²⁺ in water as low as 1 × 10^-5 M for B and 5 × 10^-5 M for A, making these complexes good candidates to be used as real-time Hg²⁺ detectors. The nature of the interaction of the Pt₂ half-lantern complex A with the Hg²⁺ cation, has been investigated by theoretical calculations