Thiabendazole (TBZ) is a benzimidazole derivative that is widely used as a fungicide. It is used to control fruit and vegetable diseases such as mold, rot, blight and spots. It is widely used in bananas and citrus fruits to maintain freshness and as a component of wax that is applied to the fruit. According to the above, it is mostly retained in the fruit peel. TBZ has low toxicity, but it is classified as a carcinogen at higher doses and can cause thyroid disorders. In the human body, when consuming fruits and vegetables treated with TBZ, in accordance with existing regulations, a concentration of TBZ will be insufficient to cause cancer. The most commonly used methods for TBZ determination are chromatographic methods, Raman spectroscopy, UV/Vis spectrophotometry, phosphorimetry and immunoassay. Most of the mentioned techniques require expensive equipment, the use of large amounts of organic solvents, and complicated procedures and sample preparations. Considering the above, the use of ion selective electrodes (ISE) is a good alternative method due to the simplicity of their preparation, application and good analytical performance. Such electrodes usually contain a liquid membrane containing sensor material, plasticizer and polyvinyl chloride. The main disadvantage of electrodes with a liquid membrane is the possibility of leakage of the sensor material. The aforementioned deficiency leads to changes in the characteristics of the sensor itself and shortened lifetime of the sensor. In order to avoid the mentioned shortcomings, it is possible to use solid-state sensors or to modify the composition of the membrane using sensor materials that are as less soluble as possible in water. Carbon-based nanomaterials have proven to be very suitable for this purpose. Modification of the sensor material with multi-walled carbon nanotubes (MWCNT) results in better response characteristics of the sensor, less leaching of the sensor material from the membrane and less signal noise. MWCNTs can be covalently modified while retaining their initial properties, thus forming a hybrid sensor material. Direct potentiometric measurements, using sensors with a solid contact and a liquid membrane modified with MWCNT, have great potential for TBZ determination due to the simplicity of the method and the simple application, accuracy and durability of such sensors. In this work, a new potentiometric solid-state sensor with a liquid membrane was used to determine TBZ. A hybrid material based on MWCNT modified with sulfate group and TBZ ion was used as the sensor material of the new sensor. The sensor showed a Nernstian response for TBZ (60.4 mV/decade of activity) in the measuring range between 8.6 ∙ 10-7 and 1.0 ∙ 10-3 M. The detection limit for TBZ was 6.2 ∙ 10-7 while the response time of the sensor for TBZ was 8 s. The signal drift was 1.7 mV/h. The measuring pH range of the sensor was between 2 and 4, which was not a limitation for the use of the sensor because TBZ is soluble only at pH 2.6. The sensor was successfully applied for direct potentiometric determination of TBZ in pure solutions and in complex real samples (peel of orange, lemon, banana, clementine and lime). The accuracy of the sensor was confirmed using the standard addition method. The lifetime of the sensor, with daily use, was approximately three months.