We describe the synthesis, properties, and application of a new fluorescent potassium chemosensor, KS2, for K+ sensing and imaging in live cells. By virtue of a strong electron-withdrawing group, 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF), with a triazacryptand ligand, the new sensor can respond to K+ up to 1.6 M. This is the first highly selective intracellular sensor suitable for sensing K+ over a broad and high concentration range. Confocal fluorescence microscopy has established the utility of KS2 for live-cell K+ detection. The application of KS2 combined with other sensors will be of great benefit for investigating cellular metabolism, detecting and diagnosing diseases including cancer, and monitoring responses to therapy.