Ion selective optical nanosensors allow accurate ion measurements in biological systems, without the physical limitations and invasiveness of ion selective electrodes. Optically based nanosensors (Photonic Explorers for Bioanalysis with Biologically Localized Embedding, PEBBLEs), have been optimized for fluorescence microscopy imaging, and have been applied for imaging various biochemical analytes. In here, we report the first example of a potassium selective nanosensor optimized for photoacoustic (PA) imaging. Notably, PA imaging overcomes the severe light penetration depth problem faced by fluorescence imaging in vivo. The new potassium selective nanosensor shows excellent response in the biological range, from 0 to 200 mM, as confirmed by both UV-Vis Spectroscopy and PA Spectroscopy. Furthermore, the K+ PEBBLE showed a 2 orders of magnitude, or higher, selectivity to K+ , relative to any other biological cations, such as Li+, Na+, Ca2+, and Mg2+.