New potentiometric membrane sensors have been designed to measure Th4+ ions using two derivatives of 18-crown-6-ether as neutral ion-carriers. The membrane composition was optimized through a multi-stage process regarding plasticizers, lipophilic ionic additives, and ion-carrier content, and the corresponding sensors' performance characteristics were investigated. The optimized sensors exhibited a Nernstian behavior for Th4+ ions over wide working concentration and pH ranges. The sensors' repeatability, reproducibility, and reversibility were tested under optimal conditions. The selectivity studies indicated the selective behavior of the developed sensors towards Th4+ ions with respect to various mono, di, and trivalent ions. The sensors possess a fast response time and can work for at least 12 weeks without significant changes in their sensitivity. Th4+ ions concentration in water samples was directly assessed by the developed sensors, with good recovery and relative standard deviation values indicating the developed sensors' accuracy and precision. Moreover, the obtained results were in good agreement with those obtained by the spectrometric method.