The purpose of this study is to develop a sensor for measuring the water content in concrete and cement mortar elements usable in fire tests. Annealed copper wires were used as electrodes of the water content sensor. Each electrode is 20 mm in length and 0.8 mm in diameter. The separation distance between the electrodes is 2 mm. By measuring the electric resistance, water content can be monitored continuously. Mortar bar specimens were used to calibrate the sensor by measuring electric resistance as a function of water content at a constant temperature of 26°C. The temperature dependence of the electrical resistance was approximated by a functional relationship developed by Ichinose for a similar type of sensor. As a result, a calibration formula was derived for electrical resistances in the range of 1.51 to 2330 kΩ, temperatures in the range of 10 to 175°C, and volumetric water content in the range of 0.084 to 0.201 m3/m3. To verify the applicability, the sensors were embedded in a wall specimen heated by ISO 834 fire for 30 minutes. As a result, it was possible to measure the water contents continuously.