Fluorescent sensor for the perrhenate/pertechnetate anions in aqueous media are rarely reported in literature as their large size and low charge density poses great challenge to their detection. Herein, we report the first fluorescent turn-on ratiometric sensor for perrhenate anion, exploiting its weak hydration property which leads to an advantageous formation of a contact ion-pair of perrhenate anion with a well-known cationic ultrafast molecular rotor dye, Auramine O. This ion-pair formation, in turn, leads to a fluorescent aggregate assembly that provides an excellent opportunity to detect perrhenate anion in aqueous solution.