Monitoring body temperature has the potential to evaluate physical conditions. The oral cavity has a large accessible volume and exhibits a temperature close to the deep body temperature; the wireless monitoring of intra-oral temperature is also possible. However, devices must be small enough to not interfere with tongue movement. Here, we fabricated a mouthguard (MG) thermometer that housed a telemeter, a thermistor, and a battery on the buccal side surface of the molar and premolar of the upper jaw. The devices were sealed between two layers of MG material for waterproofing. Sealing the devices induced a longer response time to temperature changes (from 3.2 to 12.2 s), but the quantitative characteristics did not change. Rapid changes in intra-oral temperature were successfully monitored when ingesting water of various temperatures by using the MG thermometer. Moreover, a high correlation between the difference in the intra-oral and water temperatures and the intra-oral temperature change was observed. In addition, stable wireless measurement was possible during 3 h in an office with the wearer engaged in normal activities. The effects of changes in ambient temperature and the wearing of face masks on intra-oral and skin temperatures were also investigated, where an effect of ambient temperature on skin temperature but not on intra-oral temperature was observed. These results suggest that wireless temperature monitoring in the oral cavity is feasible.