A new Tm(III) ion-selective liquid polymeric membrane sensor based on N-[(E)-1-(2- thienyl)methylidene]-N-(4-{[(Z)-1-(2-thienyl)methylidene]amino}phenyl)amine (TTPA) has been developed. When optimized, it displays a linear Nernstian response over the range of 1.0×10-7 - 1×10-2 M of Tm(III) ion, with a slope of 20.1 mV per decade and detection limit was 8.6 ×10-8 M. The effect of several parameters, such as: a) content of the ionophore in the membrane, b) kind of the plasticizer in the membrane, c) content of additive in the membrane, d) pH of the sample solution, e) response time of the sensor and f) potentiometric selectivity coefficient were investigated. The potentiometric selectivity coefficients of thulium sensor was evaluated using the matched potential method. The sensor displayed a good selectivity for thulium with respect to number of common foreign metal ions. In order to investigate the sensors analytical applicability, it was tested as an indicator electrode in potentiometric titration of thulium against EDTA and to the determination of fluoride ions in mouth wash samples.