We synthesized an indanedione–coumarin dyad (1) and used it as an off–on type fluorescent receptor for rapid, selective, and sensitive detection of CN⁻ in an aqueous mixture. The receptor itself shows very weak fluorescence from the coumarin moiety due to the photoinduced electron transfer (PET) from the photoexcited coumarin to the indanedione moiety. Selective nucleophilic addition of CN⁻ to the β-carbon of the receptor suppresses the PET and results in strong fluorescence enhancement. The reaction mechanism was fully confirmed by ¹H, ¹³C NMR, Job's plot, and FAB-MS analysis. The receptor successfully detects CN⁻ as low as 25 μM within 20 min.