Potassium ions (K) within the tumor microenvironment, along with dysregulation of K channels, play critical roles in supporting cancer cell survival and preventing their elimination. Directly monitoring changes in K homeostasis within cancer cells is invaluable for understanding these processes. However, achieving high selectivity over other biological metal ions, a detection dynamic range that aligns with intracellular K levels, and broad accessibility to research laboratories remain technically challenging for current K imaging probes. In this study, we report the selection of the first K-specific RNA-cleaving DNAzyme and the development of a K-specific DNAzyme fluorescent sensor with exceptional selectivity, achieving over 1000-fold selectivity against Na and more than 100-fold selectivity over other major biologically relevant metal ions. This sensor has an apparent dissociation constant (105 mM) that is close to the intracellular level of K, and it has a broad detection range from 21 to 200 mM K. Using this tool, we reveal a progressive decline in intracellular K levels in breast cancer cells with more advanced progression states. Moreover, we demonstrate that elevated extracellular K levels interfere with the efficacy of anticancer compounds like ML133 and Amiodarone, suggesting an underappreciated role of microenvironmental K in chemoresistance. Notably, blocking the Kir2.1 channel activity restored treatment sensitivity, presenting a potential strategy to overcome chemoresistance in aggressive cancers. These findings underscore the role of K homeostasis in tumor progression and support further exploration of ion-channel-targeted cancer therapies.