A terbium complex with long luminescence lifetime for the selective time-gated detection of potassium is presented. The complex demonstrates high sensitivity with a 25-fold increase in time-gated luminescence intensity between 0 and 15 mM potassium. The high selectivity of the complex for potassium over other physiological cations, and especially sodium (93 fold), is likely due to a favorable cation-pi interaction between a phenol moiety and the potassium ion.