A temperature optical fiber sensor based on 2,4,5-triphenylimidazole, also known as Lophine, with thermochromic and hydrophobic properties, is presented. To our knowledge this material has never been used before in this kind of application. The sensor is a low-cost device with a simple fabrication process. Its operation requires inexpensive LED sources and standard multimode fibers. Furthermore, it has been designed for specific immersed applications, e.g., in fish bowls, nurseries, or washing machines. Experimental results in different environmental conditions are presented and discussed. The fabricated sensors show a sensitivity of 0.06 dB/°C with no sign of hysteresis and no cross-sensitivity to humidity. The work is completed with a brief study of the thermochromic material.